Part of a parabola (blue), with various features (other colours). The complete parabola has no endpoints. In this orientation, it extends infinitely to the left, right, and upward.
The parabola is a member of the family of conic sections.

In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several other superficially different mathematical descriptions, which can all be proved to define exactly the same curves.

One description of a parabola involves a point (the focus) and a line (the directrix). The focus does not lie on the directrix. The parabola is the locus of points in that plane that are equidistant from both the directrix and the focus. Another description of a parabola is as a conic section, created from the intersection of a right circular conical surface and a plane parallel to another plane that is tangential to the conical surface.[a]

The line perpendicular to the directrix and passing through the focus (that is, the line that splits the parabola through the middle) is called the "axis of symmetry". The point where the parabola intersects its axis of symmetry is called the "vertex" and is the point where the parabola is most sharply curved. The distance between the vertex and the focus, measured along the axis of symmetry, is the "focal length". The "latus rectum" is the chord of the parabola that is parallel to the directrix and passes through the focus. Parabolas can open up, down, left, right, or in some other arbitrary direction. Any parabola can be repositioned and rescaled to fit exactly on any other parabola—that is, all parabolas are geometrically similar.

Parabolas have the property that, if they are made of material that reflects light, then light that travels parallel to the axis of symmetry of a parabola and strikes its concave side is reflected to its focus, regardless of where on the parabola the reflection occurs. Conversely, light that originates from a point source at the focus is reflected into a parallel ("collimated") beam, leaving the parabola parallel to the axis of symmetry. The same effects occur with sound and other waves. This reflective property is the basis of many practical uses of parabolas.

The parabola has many important applications, from a parabolic antenna or parabolic microphone to automobile headlight reflectors and the design of ballistic missiles. They are frequently used in physics, engineering, and many other areas.

Contents

HistoryEdit

 
Parabolic compass designed by Leonardo da Vinci

The earliest known work on conic sections was by Menaechmus in the 4th century BC. He discovered a way to solve the problem of doubling the cube using parabolas. (The solution, however, does not meet the requirements of compass-and-straightedge construction.) The area enclosed by a parabola and a line segment, the so-called "parabola segment", was computed by Archimedes by the method of exhaustion in the 3rd century BC, in his The Quadrature of the Parabola. The name "parabola" is due to Apollonius, who discovered many properties of conic sections. It means "application", referring to "application of areas" concept, that has a connection with this curve, as Apollonius had proved.[1] The focus–directrix property of the parabola and other conic sections is due to Pappus.

Galileo showed that the path of a projectile follows a parabola, a consequence of uniform acceleration due to gravity.

The idea that a parabolic reflector could produce an image was already well known before the invention of the reflecting telescope.[2] Designs were proposed in the early to mid-17th century by many mathematicians, including René Descartes, Marin Mersenne,[3] and James Gregory.[4] When Isaac Newton built the first reflecting telescope in 1668, he skipped using a parabolic mirror because of the difficulty of fabrication, opting for a spherical mirror. Parabolic mirrors are used in most modern reflecting telescopes and in satellite dishes and radar receivers.[5]

Definition as a locus of pointsEdit

A parabola can be defined geometrically as a set of points (locus of points) in the Euclidean plane:

  • A parabola is a set of points, such that for any point   of the set the distance   to a fixed point  , the focus, is equal to the distance   to a fixed line  , the directrix:
 

The midpoint   of the perpendicular from the focus   onto the directrix   is called vertex, and the line   is the axis of symmetry of the parabola.

In a cartesian coordinate systemEdit

Axis of symmetry parallel to the y axisEdit

 
Parabola: Definition, p: semi-latus rectum
 
Parabola: axis parallel to y axis
 
Parabola: general case

If one introduces Cartesian coordinates, such that   and the directrix has the equation  , one obtains for a point   from   the equation  . Solving for   yields

 

This parabola is U-shaped (opening to the top).

The horizontal chord through the focus (see picture in opening section) is called the latus rectum; one half of it is the semi-latus rectum. The latus rectum is parallel to the directrix. The semi-latus rectum is designated by the letter  . From the picture one obtains

 

The latus rectum is defined similarly for the other two conics – the ellipse and the hyperbola. The latus rectum is the line drawn through a focus of a conic section parallel to the directrix and terminated both ways by the curve. For any case,   is the radius of the osculating circle at the vertex. For a parabola, the semi-latus rectum,  , is the distance of the focus from the directrix. Using the parameter  , the equation of the parabola can be rewritten as

 

More generally, if the vertex is  , the focus  , and the directrix  , one obtains the equation

 
Remarks
  1. In the case of   the parabola has a downward opening.
  2. The presumption that the axis is parallel to the y axis allows one to consider a parabola as the graph of a polynomial of degree 2, and conversely: the graph of an arbitrary polynomial of degree 2 is a parabola (see next section).
  3. If one exchanges   and  , one obtains equations of the form  . These parabolas open to the left (if  ) or to the right (if  ).

General caseEdit

If the focus is  , and the directrix  , then one obtains the equation

 

(the left side of the equation uses the Hesse normal form of a line to calculate the distance  ).

For a parametric equation of a parabola in general position see § As the affine image of the unit parabola.

The implicit equation of a parabola is defined by an irreducible polynomial of degree two:

 

such that   or, equivalently, such that   is the square of a linear polynomial.

As a graph of a functionEdit

 
Parabolas  

The previous section shows that any parabola with the origin as vertex and the y axis as axis of symmetry can be considered as the graph of a function

 

For   the parabolas are opening to the top, and for   are opening to the bottom (see picture). From the section above one obtains:

  • The focus is  ,
  • the focal length  , the semi-latus rectum is  ,
  • the vertex is  ,
  • the directrix has the equation  ,
  • the tangent at point   has the equation  .

For   the parabola is the unit parabola with equation  . Its focus is  , the semi-latus rectum  , and the directrix has the equation  .

The general function of degree 2 is

 .

Completing the square yields

 

which is the equation of a parabola with

  • the axis   (parallel to the y axis),
  • the focal length  , the semi-latus rectum  ,
  • the vertex  ,
  • the focus  ,
  • the directrix  ,
  • the point of the parabola intersecting the y axis has coordinates  ,
  • the tangent at a point on the y axis has the equation  .

Similarity to the unit parabolaEdit

 
When the parabola   is uniformly scaled by factor 2, the result is the parabola  

Two objects in the Euclidean plane are similar if one can be transformed to the other by a similarity, that is, an arbitrary composition of rigid motions (translations and rotations) and uniform scalings.

A parabola   with vertex   can be transformed by the translation   to one with the origin as vertex. A suitable rotation around the origin can then transform the parabola to one that has the y axis as axis of symmetry. Hence the parabola   can be transformed by a rigid motion to a parabola with an equation  . Such a parabola can then be transformed by the uniform scaling   into the unit parabola with equation  . Thus, any parabola can be mapped to the unit parabola by a similarity.[6]

A synthetic approach, using similar triangles, can also be used to establish this result.[7]

The general result is that two conic sections (necessarily of the same type) are similar if and only if they have the same eccentricity.[6] Thus, only circles (all having eccentricity 0) share this property with parabolas (all having eccentricity 1), while general ellipses and hyperbolas do not.

There are other simple affine transformations that map the parabola   onto the unit parabola, such as  . But this mapping is not a similarity, and only shows that all parabolas are affinely equivalent (see § As the affine image of the unit parabola).

As a special conic sectionEdit

 
Pencil of conics with a common vertex

The pencil of conic sections with the x axis as axis of symmetry, one vertex at the origin (0, 0) and the same semi-latus rectum   can be represented by the equation

 

with   the eccentricity.

  • For   the conic is a circle (osculating circle of the pencil),
  • for   an ellipse,
  • for   the parabola with equation  
  • for   a hyperbola (see picture).

In polar coordinatesEdit

 
Pencil of conics with a common focus

If p > 0, the parabola with equation   (opening to the right) has the polar representation

 
( ).

Its vertex is  , and its focus is  .

If one shifts the origin into the focus, that is,  , one obtains the equation

 

Remark 1: Inverting this polar form shows that a parabola is the inverse of a cardioid.

Remark 2: The second polar form is a special case of a pencil of conics with focus   (see picture):

  (  is the eccentricity).

Conic section and quadratic formEdit

Diagram, description, and definitionsEdit

 
Cone with cross-sections

The diagram represents a cone with its axis AV. The point A is its apex. An inclined cross-section of the cone, shown in pink, is inclined from the axis by the same angle θ, as the side of the cone. According to the definition of a parabola as a conic section, the boundary of this pink cross-section EPD is a parabola.

A cross-section perpendicular to the axis of the cone passes through the vertex P of the parabola. This cross-section is circular, but appears elliptical when viewed obliquely, as is shown in the diagram. Its centre is V, and PK is a diameter. We will call its radius r.

Another perpendicular to the axis, circular cross-section of the cone is farther from the apex A than the one just described. It has a chord DE, which joins the points where the parabola intersects the circle. Another chord BC is the perpendicular bisector of DE and is consequently a diameter of the circle. These two chords and the parabola's axis of symmetry PM all intersect at the point M.

All the labelled points, except D and E, are coplanar. They are in the plane of symmetry of the whole figure. This includes the point F, which is not mentioned above. It is defined and discussed below, in § Position of the focus.

Let us call the length of DM and of EM x, and the length of PM y.

Derivation of quadratic equationEdit

The lengths of BM and CM are:

  (triangle BPM is isosceles, because  ),
  (PMCK is a parallelogram).

Using the intersecting chords theorem on the chords BC and DE, we get

 

Substituting:

 

Rearranging:

 

For any given cone and parabola, r and θ are constants, but x and y are variables that depend on the arbitrary height at which the horizontal cross-section BECD is made. This last equation shows the relationship between these variables. They can be interpreted as Cartesian coordinates of the points D and E, in a system in the pink plane with P as its origin. Since x is squared in the equation, the fact that D and E are on opposite sides of the y axis is unimportant. If the horizontal cross-section moves up or down, toward or away from the apex of the cone, D and E move along the parabola, always maintaining the relationship between x and y shown in the equation. The parabolic curve is therefore the locus of points where the equation is satisfied, which makes it a Cartesian graph of the quadratic function in the equation.

Focal lengthEdit

It is proved in a preceding section that if a parabola has its vertex at the origin, and if it opens in the positive y direction, then its equation is y = x2/4f, where f is its focal length.[b] Comparing this with the last equation above shows that the focal length of the parabola in the cone is r sin θ.

Position of the focusEdit

In the diagram above, the point V is the foot of the perpendicular from the vertex of the parabola to the axis of the cone. The point F is the foot of the perpendicular from the point V to the plane of the parabola.[c] By symmetry, F is on the axis of symmetry of the parabola. Angle VPF is complementary to θ, and angle PVF is complementary to angle VPF, therefore angle PVF is θ. Since the length of PV is r, the distance of F from the vertex of the parabola is r sin θ. It is shown above that this distance equals the focal length of the parabola, which is the distance from the vertex to the focus. The focus and the point F are therefore equally distant from the vertex, along the same line, which implies that they are the same point. Therefore, the point F, defined above, is the focus of the parabola.

This discussion started from the definition of a parabola as a conic section, but it has now led to a description as a graph of a quadratic function. This shows that these two descriptions are equivalent. They both define curves of exactly the same shape.

Alternative proof with Dandelin spheresEdit

 
Parabola (red): side projection view and top projection view of a cone with a Dandelin sphere

An alternative proof can be done using Dandelin spheres. It works without calculation and uses elementary geometric considerations only (see the derivation below).

The intersection of an upright cone by a plane  , whose inclination from vertical is the same as a generatrix (a.k.a. generator line, a line containing the apex and a point on the cone surface)   of the cone, is a parabola (red curve in the diagram).

This generatrix   is the only generatrix of the cone that is parallel to plane  . Otherwise, if there are two generatrices parallel to the intersecting plane, the intersection curve will be a hyperbola (or degenerate hyperbola, if the two generatrices are in the intersecting plane). If there is no generatrix parallel to the intersecting plane, the intersection curve will be an ellipse or a circle (or a point).

Let plane   be the plane that contains the vertical axis of the cone and line  . The inclination of plane   from vertical is the same as line   means that, viewing from the side (that is, the plane   is perpendicular to plane  ),  .

In order to prove the directrix property of a parabola (see § Definition as a locus of points above), one uses a Dandelin sphere  , which is a sphere that touches the cone along a circle   and plane   at point  . The plane containing the circle   intersects with plane   at line  . There is a mirror symmetry in the system consisting of plane  , Dandelin sphere   and the cone (the plane of symmetry is  ).

Since the plane containing the circle   is perpendicular to plane  , and  , their intersection line   must also be perpendicular to plane  . Since line   is in plane  ,  .

It turns out that   is the focus of the parabola, and   is the directrix of the parabola.

  1. Let   be an arbitrary point of the intersection curve.
  2. The generatrix of the cone containing   intersects circle   at point  .
  3. The line segments   and   are tangential to the sphere  , and hence are of equal length.
  4. Generatrix   intersects the circle   at point  . The line segments   and   are tangential to the sphere  , and hence are of equal length.
  5. Let line   be the line parallel to   and passing through point  . Since  , and point   is in plane  , line   must be in plane  . Since  , we know that   as well.
  6. Let point   be the foot of the perpendicular from point   to line  , that is,   is a segment of line  , and hence  .
  7. From intercept theorem and   we know that  . Since  , we know that  , which means that the distance from   to the focus   is equal to the distance from   to the directrix  .

Proof of the reflective propertyEdit

 
Reflective property of a parabola

The reflective property states that if a parabola can reflect light, then light that enters it travelling parallel to the axis of symmetry is reflected toward the focus. This is derived from geometrical optics, based on the assumption that light travels in rays. In the following proof, the fact that every point on the parabola is equidistant from the focus and from the directrix is taken as axiomatic.

Consider the parabola y = x2. Since all parabolas are similar, this simple case represents all others. The right side of the diagram shows part of this parabola.

Construction and definitionsEdit

The point E is an arbitrary point on the parabola, with coordinates (x, x2). The focus is F, the vertex is A (the origin), and the line FA (the y axis) is the axis of symmetry. The line EC is parallel to the axis of symmetry and intersects the x axis at D. The point C is located on the directrix (which is not shown, to minimize clutter). The point B is the midpoint of the line segment FC.

DeductionsEdit

Measured along the axis of symmetry, the vertex A is equidistant from the focus F and from the directrix. According to the intercept theorem, since C is on the directrix, the y coordinates of F and C are equal in absolute value and opposite in sign. B is the midpoint of FC, so its y coordinate is zero, thus it lies on the x axis. Its x coordinate is half that of E, D, and C, that is, x/2. The slope of the line BE is the quotient of the lengths of ED and BD, which is x2/x/2 = 2x. But 2x is also the slope (first derivative) of the parabola at E. Therefore, the line BE is the tangent to the parabola at E.

The distances EF and EC are equal because E is on the parabola, F is the focus and C is on the directrix. Therefore, since B is the midpoint of FC, triangles △FEB and △CEB are congruent (three sides), which implies that the angles marked α are congruent. (The angle above E is vertically opposite angle ∠BEC.) This means that a ray of light that enters the parabola and arrives at E travelling parallel to the axis of symmetry will be reflected by the line BE so it travels along the line EF, as shown in red in the diagram (assuming that the lines can somehow reflect light). Since BE is the tangent to the parabola at E, the same reflection will be done by an infinitesimal arc of the parabola at E. Therefore, light that enters the parabola and arrives at E travelling parallel to the axis of symmetry of the parabola is reflected by the parabola toward its focus.

The point E has no special characteristics. This conclusion about reflected light applies to all points on the parabola, as is shown on the left side of the diagram. This is the reflective property.

Other consequencesEdit

There are other theorems that can be deduced simply from the above argument.

Tangent bisection propertyEdit

The above proof and the accompanying diagram show that the tangent BE bisects the angle ∠FEC. In other words, the tangent to the parabola at any point bisects the angle between the lines joining the point to the focus and perpendicularly to the directrix.

Intersection of a tangent and perpendicular from focusEdit

 
Perpendicular from focus to tangent

Since triangles △FBE and △CBE are congruent, FB is perpendicular to the tangent BE. Since B is on the x axis, which is the tangent to the parabola at its vertex, it follows that the point of intersection between any tangent to a parabola and the perpendicular from the focus to that tangent lies on the line that is tangential to the parabola at its vertex. See animated diagram[8] and pedal curve.

Reflection of light striking the convex sideEdit

If light travels along the line CE, it moves parallel to the axis of symmetry and strikes the convex side of the parabola at E. It is clear from the above diagram that this light will be reflected directly away from the focus, along an extension of the segment FE.

Alternative proofsEdit

 
Parabola and tangent

The above proofs of the reflective and tangent bisection properties use a line of calculus. Here a geometric proof is presented.

In this diagram, F is the focus of the parabola, and T and U lie on its directrix. P is an arbitrary point on the parabola. PT is perpendicular to the directrix, and the line MP bisects angle ∠FPT. Q is another point on the parabola, with QU perpendicular to the directrix. We know that FP = PT and FQ = QU. Clearly, QT > QU, so QT > FQ. All points on the bisector MP are equidistant from F and T, but Q is closer to F than to T. This means that Q is to the left of MP, that is, on the same side of it as the focus. The same would be true if Q were located anywhere else on the parabola (except at the point P), so the entire parabola, except the point P, is on the focus side of MP. Therefore, MP is the tangent to the parabola at P. Since it bisects the angle ∠FPT, this proves the tangent bisection property.

The logic of the last paragraph can be applied to modify the above proof of the reflective property. It effectively proves the line BE to be the tangent to the parabola at E if the angles α are equal. The reflective property follows as shown previously.

Pin and string constructionEdit

 
Parabola: pin string construction

The definition of a parabola by its focus and directrix can be used for drawing it with help of pins and strings:[9]

  1. Choose the focus   and the directrix   of the parabola.
  2. Take a triangle of a set square and prepare a string with length   (see diagram).
  3. Pin one end of the string at point   of the triangle and the other one to the focus  .
  4. Position the triangle such that the second edge of the right angle is free to slide along the directrix.
  5. Take a pen and hold the string tight to the triangle.
  6. While moving the triangle along the directrix, the pen draws an arc of a parabola, because of   (see definition of a parabola).

Properties related to Pascal's theoremEdit

A parabola can be considered as the affine part of a non-degenerated projective conic with a point   on the line of infinity  , which is the tangent at  . The 5-, 4- and 3- point degenerations of Pascal's theorem are properties of a conic dealing with at least one tangent. If one considers this tangent as the line at infinity and its point of contact as the point at infinity of the y axis, one obtains three statements for a parabola.

The following properties of a parabola deal only with terms connect, intersect, parallel, which are invariants of similarities. So, it is sufficient to prove any property for the unit parabola with equation  .

4-points propertyEdit

 
4-points property of a parabola

Any parabola can be described in a suitable coordinate system by an equation  .

  • Let   be four points of the parabola  , and   the intersection of the secant line   with the line   and let   be the intersection of the secant line   with the line   (see picture). Then the secant line   is parallel to line  .
(The lines   and   are parallel to the axis of the parabola.)

Proof: straightforward calculation for the unit parabola  .

Application: The 4-points property of a parabola can be used for the construction of point  , while   and   are given.

Remark: the 4-points property of a parabola is an affine version of the 5-point degeneration of Pascal's theorem.

3-points–1-tangent propertyEdit

 
3-points–1-tangent property

Let   be three points of the parabola with equation   and   the intersection of the secant line   with the line   and   the intersection of the secant line   with the line   (see picture). Then the tangent at point   is parallel to the line  . (The lines   and   are parallel to the axis of the parabola.)

Proof: can be performed for the unit parabola  . A short calculation shows: line   has slope   which is the slope of the tangent at point  .

Application: The 3-points-1-tangent-property of a parabola can be used for the construction of the tangent at point  , while   are given.

Remark: The 3-points-1-tangent-property of a parabola is an affine version of the 4-point-degeneration of Pascal's theorem.

2-points–2-tangents propertyEdit

 
2-points–2-tangents property

Let   be two points of the parabola with equation  , and   the intersection of the tangent at point   with the line  , and   the intersection of the tangent at point   with the line   (see picture). Then the secant   is parallel to the line  . (The lines   and   are parallel to the axis of the parabola.)

Proof: straight forward calculation for the unit parabola  .

Application: The 2-points–2-tangents property can be used for the construction of the tangent of a parabola at point  , if   and the tangent at   are given.

Remark 1: The 2-points–2-tangents property of a parabola is an affine version of the 3-point degeneration of Pascal's theorem.

Remark 2: The 2-points–2-tangents property should not be confused with the following property of a parabola, which also deals with 2 points and 2 tangents, but is not related to Pascal's theorem.

Axis directionEdit

 
Construction of the axis direction

The statements above presume the knowledge of the axis direction of the parabola, in order to construct the points  . The following property determines the points   by two given points and their tangents only, and the result is that the line   is parallel to the axis of the parabola.

Let

  1.   be two points of the parabola  , and   be their tangents;
  2.   be the intersection of the tangents  ,
  3.   be the intersection of the parallel line to   through   with the parallel line to   through   (see picture).

Then the line   is parallel to the axis of the parabola and has the equation  

Proof: can be done (like the properties above) for the unit parabola  .

Application: This property can be used to determine the direction of the axis of a parabola, if two points and their tangents are given. An alternative way is to determine the midpoints of two parallel chords, see section on parallel chords.

Remark: This property is an affine version of the theorem of two perspective triangles of a non-degenerate conic.[10]

Steiner generationEdit

ParabolaEdit

 
Steiner generation of a parabola

Steiner established the following procedure for the construction of a non-degenerate conic (see Steiner conic):

  • Given two pencils   of lines at two points   (all lines containing   and   respectively) and a projective but not perspective mapping   of   onto  , the intersection points of corresponding lines form a non-degenerate projective conic section.

This procedure can be used for a simple construction of points on the parabola  :

  • Consider the pencil at the vertex   and the set of lines   that are parallel to the y axis.
  1. Let   be a point on the parabola, and  ,  .
  2. The line segment   is divided into n equally spaced segments, and this division is projected (in the direction  ) onto the line segment   (see figure). This projection gives rise to a projective mapping   from pencil   onto the pencil  .
  3. The intersection of the line   and the i-th parallel to the y axis is a point on the parabola.

Proof: straightforward calculation.

Remark: Steiner's generation is also available for ellipses and hyperbolas.

Dual parabolaEdit

 
Dual parabola and Bezier curve of degree 2 (right: curve point and division points   for parameter  )

A dual parabola consists of the set of tangents of an ordinary parabola.

The Steiner generation of a conic can be applied to the generation of a dual conic by changing the meanings of points and lines:

  • Let be given two point sets on two lines  , and a projective but not perspective mapping   between these point sets, then the connecting lines of corresponding points form a non degenerate dual conic.

In order to generate elements of a dual parabola, one starts with

  1. three points   not on a line,
  2. divides the line sections   and   each into   equally spaced line segments and adds numbers as shown in the picture.
  3. Then the lines   are tangents of a parabola, hence elements of a dual parabola.
  4. The parabola is a Bezier curve of degree 2 with the control points  .

The proof is a consequence of the de Casteljau algorithm for a Bezier curve of degree 2.

Inscribed angles and the 3-point formEdit

 
Inscribed angles of a parabola

A parabola with equation   is uniquely determined by three points   with different x coordinates. The usual procedure to determine the coefficients   is to insert the point coordinates into the equation. The result is a linear system of three equations, which can be solved by Gaussian elimination or Cramer's rule, for example. An alternative way uses the inscribed angle theorem for parabolas.

In the following, the angle of two lines will be measured by the difference of the slopes of the line with respect to the directrix of the parabola. That is, for a parabola of equation   the angle between two lines of equations   is measured by  

Analogous to the inscribed angle theorem for circles, one has the inscribed angle theorem for parabolas:[11][12]

Four points   with different x coordinates (see picture) are on a parabola with equation   if and only if the angles at   and   have the same measure, as defined above. That is,
 

(Proof: straightforward calculation: If the points are on a parabola, one may translate the coordinates for having the equation  , then one has   if the points are on the parabola.)

A consequence is that the equation (in  ) of the parabola determined by 3 points   with different x coordinates is (if two x coordinates are equal, there is no parabola with directrix parallel to the x axis, which passes through the points)

 

Multiplying by the denominators that depend on   one obtains the more standard form

 

Pole–polar relationEdit

 
Parabola: pole–polar relation

In a suitable coordinate system any parabola can be described by an equation  . The equation of the tangent at a point   is

 

One obtains the function

 

on the set of points of the parabola onto the set of tangents.

Obviously, this function can be extended onto the set of all points of   to a bijection between the points of   and the lines with equations  . The inverse mapping is

line   → point  .

This relation is called the pole–polar relation of the parabola, where the point is the pole, and the corresponding line its polar.

By calculation, one checks the following properties of the pole–polar relation of the parabola:

  • For a point (pole) on the parabola, the polar is the tangent at this point (see picture:  ).
  • For a pole   outside the parabola the intersection points of its polar with the parabola are the touching points of the two tangents passing   (see picture:  ).
  • For a point within the parabola the polar has no point with the parabola in common (see picture:   and  ).
  • The intersection point of two polar lines (for example,  ) is the pole of the connecting line of their poles (in example:  ).
  • Focus and directrix of the parabola are a pole–polar pair.

Remark: Pole–polar relations also exist for ellipses and hyperbolas.

Tangent propertiesEdit

Two tangent properties related to the latus rectumEdit

Let the line of symmetry intersect the parabola at point Q, and denote the focus as point F and its distance from point Q as f. Let the perpendicular to the line of symmetry, through the focus, intersect the parabola at a point T. Then (1) the distance from F to T is 2f, and (2) a tangent to the parabola at point T intersects the line of symmetry at a 45° angle.[13]:p.26

 
Perpendicular tangents intersect on the directrix

Orthoptic propertyEdit

If two tangents to a parabola are perpendicular to each other, then they intersect on the directrix. Conversely, two tangents that intersect on the directrix are perpendicular.

Lambert's theoremEdit

Let three tangents to a parabola form a triangle. Then Lambert's theorem states that the focus of the parabola lies on the circumcircle of the triangle.[14][8]:Corollary 20

Tsukerman's converse to Lambert's theorem states that, given three lines that bound a triangle, if two of the lines are tangent to a parabola whose focus lies on the circumcircle of the triangle, then the third line is also tangent to the parabola.[15]

Facts related to chordsEdit

Focal length calculated from parameters of a chordEdit

Suppose a chord crosses a parabola perpendicular to its axis of symmetry. Let the length of the chord between the points where it intersects the parabola be c and the distance from the vertex of the parabola to the chord, measured along the axis of symmetry, be d. The focal length, f, of the parabola is given by

 
Proof

Suppose a system of Cartesian coordinates is used such that the vertex of the parabola is at the origin, and the axis of symmetry is the y axis. The parabola opens upward. It is shown elsewhere in this article that the equation of the parabola is 4fy = x2, where f is the focal length. At the positive x end of the chord, x = c/2 and y = d. Since this point is on the parabola, these coordinates must satisfy the equation above. Therefore, by substitution,  . From this,  .

Area enclosed between a parabola and a chordEdit

 
Parabola (magenta) and line (lower light blue) including a chord (blue). The area enclosed between them is in pink. The chord itself ends at the points where the line intersects the parabola.

The area enclosed between a parabola and a chord (see diagram) is two-thirds of the area of a parallelogram that surrounds it. One side of the parallelogram is the chord, and the opposite side is a tangent to the parabola.[16][17] The slope of the other parallel sides is irrelevant to the area. Often, as here, they are drawn parallel with the parabola's axis of symmetry, but this is arbitrary.

A theorem equivalent to this one, but different in details, was derived by Archimedes in the 3rd century BCE. He used the areas of triangles, rather than that of the parallelogram.[d] See The Quadrature of the Parabola.

If the chord has length b and is perpendicular to the parabola's axis of symmetry, and if the perpendicular distance from the parabola's vertex to the chord is h, the parallelogram is a rectangle, with sides of b and h. The area A of the parabolic segment enclosed by the parabola and the chord is therefore

 

This formula can be compared with the area of a triangle: 1/2bh.

In general, the enclosed area can be calculated as follows. First, locate the point on the parabola where its slope equals that of the chord. This can be done with calculus, or by using a line that is parallel to the axis of symmetry of the parabola and passes through the midpoint of the chord. The required point is where this line intersects the parabola.[e] Then, using the formula given in Distance from a point to a line, calculate the perpendicular distance from this point to the chord. Multiply this by the length of the chord to get the area of the parallelogram, then by 2/3 to get the required enclosed area.

Corollary concerning midpoints and endpoints of chordsEdit

 
Midpoints of parallel chords

A corollary of the above discussion is that if a parabola has several parallel chords, their midpoints all lie on a line parallel to the axis of symmetry. If tangents to the parabola are drawn through the endpoints of any of these chords, the two tangents intersect on this same line parallel to the axis of symmetry (see Axis-direction of a parabola).[f]

Arc lengthEdit

If a point X is located on a parabola with focal length f, and if p is the perpendicular distance from X to the axis of symmetry of the parabola, then the lengths of arcs of the parabola that terminate at X can be calculated from f and p as follows, assuming they are all expressed in the same units.[g]

 

This quantity s is the length of the arc between X and the vertex of the parabola.

The length of the arc between X and the symmetrically opposite point on the other side of the parabola is 2s.

The perpendicular distance p can be given a positive or negative sign to indicate on which side of the axis of symmetry X is situated. Reversing the sign of p reverses the signs of h and s without changing their absolute values. If these quantities are signed, the length of the arc between any two points on the parabola is always shown by the difference between their values of s. The calculation can be simplified by using the properties of logarithms:

 

This can be useful, for example, in calculating the size of the material needed to make a parabolic reflector or parabolic trough.

This calculation can be used for a parabola in any orientation. It is not restricted to the situation where the axis of symmetry is parallel to the y axis.

A geometrical construction to find a sector areaEdit

 

S is the focus, and V is the principal vertex of the parabola VG. Draw VX perpendicular to SV.

Take any point B on VG and drop a perpendicular BQ from B to VX. Draw perpendicular ST intersecting BQ, extended if necessary, at T. At B draw the perpendicular BJ, intersecting VX at J.

For the parabola, the segment VBV, the area enclosed by the chord VB and the arc VB, is equal to ∆VBQ / 3, also  .

The area of the parabolic sector SVB = ∆SVB + ∆VBQ / 3 .

Since triangles TSB and QBJ are similar,

 

Therefore, the area of the parabolic sector   and can be found from the length of VJ, as found above.

A circle through S, V and B also passes through J.

Conversely, if a point, B on the parabola VG is to be found so that the area of the sector SVB is equal to a specified value, determine the point J on VX and construct a circle through S, V and J. Since SJ is the diameter, the center of the circle is at its midpoint, and it lies on the perpendicular bisector of SV, a distance of one half VJ from SV. The required point B is where this circle intersects the parabola.

If a body traces the path of the parabola due to an inverse square force directed towards S, the area SVB increases at a constant rate as point B moves forward. It follows that J moves at constant speed along VX as B moves along the parabola.

If the speed of the body at the vertex where it is moving perpendicularly to SV is v, then the speed of J is equal to 3v/4.

The construction can be extended simply to include the case where neither radius coincides with the axis SV as follows. Let A be a fixed point on VG between V and B, and point H be the intersection on VX with the perpendicular to SA at A. From the above, the area of the parabolic sector  .

Conversely, if it is required to find the point B for a particular area SAB, find point J from HJ and point B as before. By Book 1, Proposition 16, Corollary 6 of Newton's Principia, the speed of a body moving along a parabola with a force directed towards the focus is inversely proportional to the square root of the radius. If the speed at A is v, then at the vertex V it is  , and point J moves at a constant speed of  .

The above construction was devised by Isaac Newton and can be found in Book 1 of Philosophiæ Naturalis Principia Mathematica as Proposition 30.

Focal length and radius of curvature at the vertexEdit

The focal length of a parabola is half of its radius of curvature at its vertex.

Proof

Consider a point (x, y) on a circle of radius R and with center at the point (0, R). The circle passes through the origin. If the point is near the origin, the Pythagorean theorem shows that

 

But if (x, y) is extremely close to the origin, since the x axis is a tangent to the circle, y is very small compared with x, so y2 is negligible compared with the other terms. Therefore, extremely close to the origin

  (1)

Compare this with the parabola

  (2)

which has its vertex at the origin, opens upward, and has focal length f (see preceding sections of this article).

Equations (1) and (2) are equivalent if R = 2f. Therefore, this is the condition for the circle and parabola to coincide at and extremely close to the origin. The radius of curvature at the origin, which is the vertex of the parabola, is twice the focal length.

Corollary

A concave mirror that is a small segment of a sphere behaves approximately like a parabolic mirror, focusing parallel light to a point midway between the centre and the surface of the sphere.

As the affine image of the unit parabolaEdit

 
Parabola as an affine image of the unit parabola

Another definition of a parabola uses affine transformations:

  • Any parabola is the affine image of the unit parabola with equation  .
parametric representation

An affine transformation of the Euclidean plane has the form  , where   is a regular matrix (determinant is not 0), and   is an arbitrary vector. If   are the column vectors of the matrix  , the unit parabola   is mapped onto the parabola

 

where

  is a point of the parabola,
  is a tangent vector at point  ,
  is parallel to the axis of the parabola (axis of symmetry through the vertex).
vertex

In general, the two vectors   are not perpendicular, and   is not the vertex, unless the affine transformation is a similarity.

The tangent vector at the point   is  . At the vertex the tangent vector is orthogonal to  . Hence the parameter   of the vertex is the solution of the equation

 

which is

 

and the vertex is

 
focal length and focus

The focal length can be determined by a suitable parameter transformation (which does not change the geometric shape of the parabola). The focal length is

 

Hence the focus of the parabola is

 
implicit representation

Solving the parametric representation for   by Cramer's rule and using  , one gets the implicit representation

 .
parabola in space

The definition of a parabola in this section gives a parametric representation of an arbitrary parabola, even in space, if one allows   to be vectors in space.

As quadratic Bézier curveEdit

 
Quadratic Bézier curve and its control points

A quadratic Bézier curve is a curve   defined by three points  ,   and  , called its control points:

 

This curve is an arc of a parabola (see § As the affine image of the unit parabola).

Numerical integrationEdit

 
Simpson's rule: the graph of a function is replaced by an arc of a parabola

In one method of numerical integration one replaces the graph of a function by arcs of parabolas and integrates the parabola arcs. A parabola is determined by three points. The formula for one arc is

 

The method is called Simpson's rule.

As plane section of quadricEdit

The following quadrics contain parabolas as plane sections:

As trisectrixEdit

 
Angle trisection with a parabola

A parabola can be used as a trisectrix, that is it allows the exact trisection of an arbitrary angle with straightedge and compass. This is not in contradiction to the impossibility of an angle trisection with compass-and-straightedge constructions alone, as the use of parabolas is not allowed in the classic rules for compass-and-straightedge constructions.

To trisect  , place its leg   on the x axis such that the vertex   is in the coordinate system's origin. The coordinate system also contains the parabola  . The unit circle with radius 1 around the origin intersects the angle's other leg  , and from this point of intersection draw the perpendicular onto the y axis. The parallel to y axis through the midpoint of that perpendicular and the tangent on the unit circle in   intersect in  . The circle around   with radius   intersects the parabola at  . The perpendicular from   onto the x axis intersects the unit circle at  , and   is exactly one third of  .

The correctness of this construction can be seen by showing that the x coordinate of   is  . Solving the equation system given by the circle around   and the parabola leads to the cubic equation  . The triple-angle formula   then shows that   is indeed a solution of that cubic equation.

This trisection goes back to René Descartes, who described it in his book La Géométrie (1637).[18]

GeneralizationsEdit

If one replaces the real numbers by an arbitrary field, many geometric properties of the parabola   are still valid:

  1. A line intersects in at most two points.
  2. At any point   the line   is the tangent.

Essentially new phenomena arise, if the field has characteristic 2 (that is,  ): the tangents are all parallel.

In algebraic geometry, the parabola is generalized by the rational normal curves, which have coordinates (x, x2, x3, …, xn); the standard parabola is the case n = 2, and the case n = 3 is known as the twisted cubic. A further generalization is given by the Veronese variety, when there is more than one input variable.

In the theory of quadratic forms, the parabola is the graph of the quadratic form x2 (or other scalings), while the elliptic paraboloid is the graph of the positive-definite quadratic form x2 + y2 (or scalings), and the hyperbolic paraboloid is the graph of the indefinite quadratic form x2y2. Generalizations to more variables yield further such objects.

The curves y = xp for other values of p are traditionally referred to as the higher parabolas and were originally treated implicitly, in the form xp = kyq for p and q both positive integers, in which form they are seen to be algebraic curves. These correspond to the explicit formula y = xp/q for a positive fractional power of x. Negative fractional powers correspond to the implicit equation xpyq = k and are traditionally referred to as higher hyperbolas. Analytically, x can also be raised to an irrational power (for positive values of x); the analytic properties are analogous to when x is raised to rational powers, but the resulting curve is no longer algebraic and cannot be analyzed by algebraic geometry.

In the physical worldEdit

In nature, approximations of parabolas and paraboloids are found in many diverse situations. The best-known instance of the parabola in the history of physics is the trajectory of a particle or body in motion under the influence of a uniform gravitational field without air resistance (for instance, a ball flying through the air, neglecting air friction).

The parabolic trajectory of projectiles was discovered experimentally in the early 17th century by Galileo, who performed experiments with balls rolling on inclined planes. He also later proved this mathematically in his book Dialogue Concerning Two New Sciences.[19][h] For objects extended in space, such as a diver jumping from a diving board, the object itself follows a complex motion as it rotates, but the center of mass of the object nevertheless moves along a parabola. As in all cases in the physical world, the trajectory is always an approximation of a parabola. The presence of air resistance, for example, always distorts the shape, although at low speeds, the shape is a good approximation of a parabola. At higher speeds, such as in ballistics, the shape is highly distorted and doesn't resemble a parabola.

Another hypothetical situation in which parabolas might arise, according to the theories of physics described in the 17th and 18th centuries by Sir Isaac Newton, is in two-body orbits, for example, the path of a small planetoid or other object under the influence of the gravitation of the Sun. Parabolic orbits do not occur in nature; simple orbits most commonly resemble hyperbolas or ellipses. The parabolic orbit is the degenerate intermediate case between those two types of ideal orbit. An object following a parabolic orbit would travel at the exact escape velocity of the object it orbits; objects in elliptical or hyperbolic orbits travel at less or greater than escape velocity, respectively. Long-period comets travel close to the Sun's escape velocity while they are moving through the inner Solar system, so their paths are nearly parabolic.

Approximations of parabolas are also found in the shape of the main cables on a simple suspension bridge. The curve of the chains of a suspension bridge is always an intermediate curve between a parabola and a catenary, but in practice the curve is generally nearer to a parabola, and in calculations the second-degree parabola is used.[20][21] Under the influence of a uniform load (such as a horizontal suspended deck), the otherwise catenary-shaped cable is deformed toward a parabola. Unlike an inelastic chain, a freely hanging spring of zero unstressed length takes the shape of a parabola. Suspension-bridge cables are, ideally, purely in tension, without having to carry other forces, for example, bending. Similarly, the structures of parabolic arches are purely in compression.

Paraboloids arise in several physical situations as well. The best-known instance is the parabolic reflector, which is a mirror or similar reflective device that concentrates light or other forms of electromagnetic radiation to a common focal point, or conversely, collimates light from a point source at the focus into a parallel beam. The principle of the parabolic reflector may have been discovered in the 3rd century BC by the geometer Archimedes, who, according to a dubious legend,[22] constructed parabolic mirrors to defend Syracuse against the Roman fleet, by concentrating the sun's rays to set fire to the decks of the Roman ships. The principle was applied to telescopes in the 17th century. Today, paraboloid reflectors can be commonly observed throughout much of the world in microwave and satellite-dish receiving and transmitting antennas.

In parabolic microphones, a parabolic reflector is used to focus sound onto a microphone, giving it highly directional performance.

Paraboloids are also observed in the surface of a liquid confined to a container and rotated around the central axis. In this case, the centrifugal force causes the liquid to climb the walls of the container, forming a parabolic surface. This is the principle behind the liquid-mirror telescope.

Aircraft used to create a weightless state for purposes of experimentation, such as NASA's "Vomit Comet", follow a vertically parabolic trajectory for brief periods in order to trace the course of an object in free fall, which produces the same effect as zero gravity for most purposes.

In the United States, vertical curves in roads are usually parabolic by design.

GalleryEdit

See alsoEdit

FootnotesEdit

  1. ^ The tangential plane just touches the conical surface along a line, which passes through the apex of the cone.
  2. ^ As stated above in the lead, the focal length of a parabola is the distance between its vertex and focus.
  3. ^ The point V is the centre of the smaller circular cross-section of the cone. The point F is in the (pink) plane of the parabola, and the line VF is perpendicular to the plane of the parabola.
  4. ^ Archimedes proved that the area of the enclosed parabolic segment was 4/3 as large as that of a triangle that he inscribed within the enclosed segment. It can easily be shown that the parallelogram has twice the area of the triangle, so Archimedes' proof also proves the theorem with the parallelogram.
  5. ^ This method can be easily proved correct by calculus. It was also known and used by Archimedes, although he lived nearly 2000 years before calculus was invented.
  6. ^ A proof of this sentence can be inferred from the proof of the orthoptic property, above. It is shown there that the tangents to the parabola y = x2 at (p, p2) and (q, q2) intersect at a point whose x coordinate is the mean of p and q. Thus if there is a chord between these two points, the intersection point of the tangents has the same x coordinate as the midpoint of the chord.
  7. ^ In this calculation, the square root q must be positive. The quantity ln a is the natural logarithm of a.
  8. ^ However, this parabolic shape, as Newton recognized, is only an approximation of the actual elliptical shape of the trajectory and is obtained by assuming that the gravitational force is constant (not pointing toward the center of the Earth) in the area of interest. Often, this difference is negligible and leads to a simpler formula for tracking motion.

CitationsEdit

  1. ^ "Can You Really Derive Conic Formulae from a Cone? – Deriving the Symptom of the Parabola – Mathematical Association of America". Retrieved 30 September 2016.
  2. ^ Wilson, Ray N. (2004). Reflecting Telescope Optics: Basic design theory and its historical development (2 ed.). Springer. p. 3. ISBN 3-540-40106-7. Extract of page 3.
  3. ^ Stargazer, p. 115.
  4. ^ Stargazer, pp. 123, 132.
  5. ^ Fitzpatrick, Richard (July 14, 2007). "Spherical Mirrors". Electromagnetism and Optics, lectures. University of Texas at Austin. Paraxial Optics. Retrieved October 5, 2011.
  6. ^ a b Kumpel, P. G. (1975), "Do similar figures always have the same shape?", The Mathematics Teacher, 68 (8): 626–628, ISSN 0025-5769.
  7. ^ Shriki, Atara; David, Hamatal (2011), "Similarity of Parabolas – A Geometrical Perspective", Learning and Teaching Mathematics, 11: 29–34.
  8. ^ a b Tsukerman, Emmanuel (2013). "On Polygons Admitting a Simson Line as Discrete Analogs of Parabolas" (PDF). Forum Geometricorum. 13: 197–208.
  9. ^ Frans van Schooten: Mathematische Oeffeningen, Leyden, 1659, p. 334.
  10. ^ Planar Circle Geometries, an Introduction to Moebius-, Laguerre- and Minkowski-planes, p. 36.
  11. ^ E. Hartmann, Lecture Note Planar Circle Geometries, an Introduction to Möbius-, Laguerre- and Minkowski Planes, p. 72.
  12. ^ W. Benz, Vorlesungen über Geomerie der Algebren, Springer (1973).
  13. ^ Downs, J. W. (2003). Practical Conic Sections. Dover Publishing.[ISBN missing]
  14. ^ Sondow, Jonathan (2013). "The parbelos, a parabolic analog of the arbelos". American Mathematical Monthly. 120: 929–935. arXiv:1210.2279. doi:10.4169/amer.math.monthly.120.10.929.
  15. ^ Tsukerman, Emmanuel (2014). "Solution of Sondow's problem: a synthetic proof of the tangency property of the parbelos". American Mathematical Monthly. 121: 438–443. arXiv:1210.5580. doi:10.4169/amer.math.monthly.121.05.438.
  16. ^ "Sovrn Container". Mathwarehouse.com. Retrieved 2016-09-30.
  17. ^ "Parabola". Mysite.du.edu. Retrieved 2016-09-30.
  18. ^ Yates, Robert C. (1941). "The Trisection Problem". National Mathematics Magazine. 15 (4): 191–202. JSTOR 3028133.
  19. ^ Dialogue Concerning Two New Sciences (1638) (The Motion of Projectiles: Theorem 1).
  20. ^ Troyano, Leonardo Fernández (2003). Bridge engineering: a global perspective. Thomas Telford. p. 536. ISBN 0-7277-3215-3.
  21. ^ Drewry, Charles Stewart (1832). A memoir of suspension bridges. Oxford University. p. 159.
  22. ^ Middleton, W. E. Knowles (December 1961). "Archimedes, Kircher, Buffon, and the Burning-Mirrors". Isis. Published by: The University of Chicago Press on behalf of The History of Science Society. 52 (4): 533–543. doi:10.1086/349498. JSTOR 228646.

Further readingEdit

  • Lockwood, E. H. (1961). A Book of Curves. Cambridge University Press.

External linksEdit