A parabola (pron.: //; plural parabolas or parabolae, adjective parabolic, from Greek: παραβολή) is a two-dimensional, mirror-symmetrical curve, which is approximately U-shaped when oriented as shown in the diagram, but which can be in any orientation in its plane. It fits any of several superficially different mathematical descriptions which can all be proved to define curves of exactly the same shape.
One description of a parabola involves a point (the focus) and a line (the directrix). The focus does not lie on the directrix. The locus of points in that plane that are equidistant from both the directrix and the focus is the parabola. Another description of a parabola is as a conic section, created from the intersection of a right circular conical surface and a plane, which is parallel to a straight line on the conical surface and perpendicular to another plane which includes both the axis of the cone and also the same straight line on its surface. A third description is algebraic. A parabola is a graph of a quadratic function, such as
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 on the axis of symmetry that intersects the parabola is called the "vertex", and it is the point where the curvature is greatest. 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 which 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 which enters a parabola travelling parallel to its axis of symmetry 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 ("collimated") into a parallel beam, leaving the parabola parallel to the axis of symmetry. The same effects occur with sound and other forms of energy. 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 to the design of ballistic missiles. They are frequently used in physics, engineering, and many other areas.
Strictly, the adjective parabolic should be applied only to things that are shaped as a parabola, which is a two-dimensional shape. However, as shown in the last paragraph, the same adjective is commonly used for three-dimensional objects, such as parabolic reflectors, which are really paraboloids. Sometimes, the noun parabola is also used to refer to these objects. Though not perfectly correct, this usage is generally understood.
The earliest known work on conic sections was by Menaechmus in the fourth century BC. He discovered a way to solve the problem of doubling the cube using parabolae. (The solution, however, does not meet the requirements imposed by compass and straightedge construction.) The area enclosed by a parabola and a line segment, the so-called "parabola segment", was computed by Archimedes via the method of exhaustion in the third 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. The focus–directrix property of the parabola and other conics 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. Designs were proposed in the early to mid seventeenth century by many mathematicians including René Descartes, Marin Mersenne, and James Gregory. 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.
Equation in Cartesian coordinates 
Let the directrix be the line x = −p and let the focus be the point (p, 0). If (x, y) is a point on the parabola then, by Pappus' definition of a parabola, it is the same distance from the directrix as the focus; in other words:
Squaring both sides and simplifying produces
as the equation of the parabola. By interchanging the roles of x and y one obtains the corresponding equation of a parabola with a vertical axis as
The equation can be generalized to allow the vertex to be at a point other than the origin by defining the vertex as the point (h, k). The equation of a parabola with a vertical axis then becomes
The last equation can be rewritten
so the graph of any function which is a polynomial of degree 2 in x is a parabola with a vertical axis.
More generally, a parabola is a curve in the Cartesian plane defined by an irreducible equation — one that does not factor as a product of two not necessarily distinct linear equations — of the general conic form
with the parabola restriction that
where all of the coefficients are real and where A and C are not both zero. The equation is irreducible if and only if the determinant of the 3×3 matrix
is non-zero: that is, if (AC − B2/4)F + BED/4 − CD2/4 − AE2/4 ≠ 0. The reducible case, also called the degenerate case, gives a pair of parallel lines, possibly real, possibly imaginary, and possibly coinciding with each other.
Conic section and quadratic form 
Cone with cross-sections
The diagram represents a cone with its axis vertical. The point A is its apex. A horizontal cross-section of the cone passes through the points B, E, C, and D. This cross-section is circular, but appears elliptical when viewed obliquely, as is shown in the diagram. An inclined cross-section of the cone, shown in pink, is inclined from the vertical 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. The cone also has another horizontal cross-section, which passes through the vertex, P, of the parabola, and is also circular, with a radius which we will call r. Its centre is V, and PK is a diameter. The chord BC is a diameter of the lower circle, and passes through the point M, which is the midpoint of the chord ED. Let us call the lengths of the line segments EM and DM x, and the length of PM y.
- (The triangle BPM is isosceles.)
- (PMCK is a parallelogram.)
Using the intersecting chords theorem on the chords BC and DE, we get:
For any given cone and parabola, r and θ are constants, but x and y are variables which depend on the arbitrary height at which the horizontal cross-section BECD is made. This last equation is a simple quadratic one which describes how x and y are related to each other, and therefore defines the shape of the parabolic curve. This shows that the definition of a parabola as a conic section implies its definition as the graph of a quadratic function. Both definitions produce curves of exactly the same shape.
Focal length 
It is proved below that if a parabola has an equation of the form y = ax2, where a is a constant, then where f is its focal length. Comparing this with the last equation above shows that the focal length of the above parabola is r sin θ.
Position of the focus 
If a line is perpendicular to the plane of the parabola and passes through the centre, V, of the horizontal cross-section of the cone passing through P, then the point where this line intersects the plane of the parabola is the focus of the parabola, which is marked F on the diagram. 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, this construction correctly places the focus on the axis of symmetry of the parabola, at a distance r sin θ from its vertex.
Other geometric definitions 
A parabola may also be characterized as a conic section with an eccentricity of 1. As a consequence of this, all parabolae are similar, meaning that while they can be different sizes, they are all the same shape. A parabola can also be obtained as the limit of a sequence of ellipses where one focus is kept fixed as the other is allowed to move arbitrarily far away in one direction. In this sense, a parabola may be considered an ellipse that has one focus at infinity. The parabola is an inverse transform of a cardioid.
A parabola has a single axis of reflective symmetry, which passes through its focus and is perpendicular to its directrix. The point of intersection of this axis and the parabola is called the vertex. A parabola spun about this axis in three dimensions traces out a shape known as a paraboloid of revolution.
The parabola is found in numerous situations in the physical world (see below).
In the following equations and are the coordinates of the vertex, , of the parabola and is the distance from the vertex to the focus and the vertex to the directrix.
Vertical axis of symmetry 
Horizontal axis of symmetry 
General parabola 
The general form for a parabola is
This result is derived from the general conic equation given below:
and the fact that, for a parabola,
The equation for a general parabola with a focus point F(u, v), and a directrix in the form
Latus rectum, semilatus rectum, and polar coordinates 
In polar coordinates, a parabola with the focus at the origin and the directrix parallel to the y-axis, is given by the equation
where l is the semilatus rectum: the distance from the focus to the parabola itself, measured along a line perpendicular to the axis of symmetry. Note that this equals the perpendicular distance from the focus to the directrix, and is twice the focal length, which is the distance from the focus to the vertex of the parabola.
The latus rectum is the chord that passes through the focus and is perpendicular to the axis of symmetry. It has a length of 2l.
Gauss-mapped form 
A Gauss-mapped form: has normal .
Proof of the reflective property 
The reflective property states that, if a parabola can reflect light, then light which enters it travelling parallel to the axis of symmetry is reflected to the focus. This is derived from the wave nature of light in the caption to a diagram near the top of this article. This derivation is valid, but may not be satisfying to readers who would prefer a mathematical approach. 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 Since all parabolas are similar, this simple case represents all others. The right-hand side of the diagram shows part of this parabola.
Construction and definitions
The point E is an arbitrary point on the parabola, with coordinates 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.
Measured along the axis of symmetry, the vertex, A, is equidistant from the focus, F, and from the directrix. Correspondingly, 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, so it lies on the x-axis. Its x-coordinate is half that of E, D, and C, i.e. The slope of the line BE is the quotient of the lengths of ED and BD, which is which comes to
But 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 are equal. (The angle above E is vertically opposite angle BEC.) This means that a ray of light which 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 infinitessimal 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.
Tangent bisection property 
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.
Alternative proofs 
The above proofs of the reflective and tangent bisection properties use a line of calculus. For readers who are not comfortable with calculus, the following alternative 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, i.e. 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 are equal. The reflective property follows as shown previously.
Two tangent properties 
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.:p.26
Orthoptic property 
If two tangents to a parabola are perpendicular to each other, then they intersect on the directrix. Conversely, two tangents which intersect on the directrix are perpendicular.
Without loss of generality, consider the parabola Suppose that two tangents contact this parabola at the points and Their slopes are and respectively. Thus the equation of the first tangent is of the form where is a constant. In order to make the line pass through the value of must be so the equation of this tangent is Likewise, the equation of the other tangent is At the intersection point of the two tangents, Thus Factoring the difference of squares, cancelling, and dividing by 2 gives Substituting this into one of the equations of the tangents gives an expression for the y-coordinate of the intersection point: Simplifying this gives
We now use the fact that these tangents are perpendicular. The product of the slopes of perpendicular lines is −1, assuming that both of the slopes are finite. The slopes of our tangents are and , so so Thus the y-coordinate of the intersection point of the tangents is given by This is also the equation of the directrix of this parabola, so the two perpendicular tangents intersect on the directrix.
Dimensions of parabolas with axes of symmetry parallel to the y-axis 
These parabolas have equations of the form By interchanging and the parabolas' axes of symmetry become parallel to the x-axis.
Some features of a parabola
Coordinates of the vertex 
The x-coordinate at the vertex is , which is found by differentiating the original equation , setting the resulting equal to zero (a critical point), and solving for . Substitute this x-coordinate into the original equation to yield:
Put terms over a common denominator
where is the discriminant,
Thus, the vertex is at point
Coordinates of the focus 
Since the axis of symmetry of this parabola is parallel with the y-axis, the x-coordinates of the focus and the vertex are equal. The coordinates of the vertex are calculated in the preceding section. The x-coordinate of the focus is therefore also
To find the y-coordinate of the focus, consider the point, P, located on the parabola where the slope is 1, so the tangent to the parabola at P is inclined at 45 degrees to the axis of symmetry. Using the reflective property of a parabola, we know that light which is initially travelling parallel to the axis of symmetry is reflected at P toward the focus. The 45-degree inclination causes the light to be turned 90 degrees by the reflection, so it travels from P to the focus along a line that is perpendicular to the axis of symmetry and to the y-axis. This means that the y-coordinate of P must equal that of the focus.
By differentiating the equation of the parabola and setting the slope to 1, we find the x-coordinate of P:
Substituting this value of in the equation of the parabola, we find the y-coordinate of P, and also of the focus:
where is the discriminant, as is used in the "Coordinates of the vertex" section.
The focus is therefore the point:
Axis of symmetry, focal length, and directrix 
The above coordinates of the focus of a parabola of the form:
can be compared with the coordinates of its vertex, which are derived in the section "Coordinates of the vertex", above, and are:
The axis of symmetry is the line which passes through both the focus and the vertex. In this case, it is vertical, with equation:
The focal length of the parabola is the difference between the y-coordinates of the focus and the vertex:
It is sometimes useful to invert this equation and use it in the form: See the section "Conic section and quadratic form", above.
Measured along the axis of symmetry, the vertex is the midpoint between the focus and the directrix. Therefore, the equation of the directrix is:
Length of an arc of a parabola 
If a point X is located on a parabola which has focal length and if is the perpendicular distance from X to the axis of symmetry of the parabola, then the lengths of arcs of the parabola which terminate at X can be calculated from and as follows, assuming they are all expressed in the same units.
This quantity, , 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
The perpendicular distance, , 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 reverses the signs of and 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 The calculation can be simplified by using the properties of logarithms:
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.
Parabolae in the physical world 
In nature, approximations of parabolae and paraboloids (such as catenary curves) 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 baseball flying through the air, neglecting air friction).
The parabolic trajectory of projectiles was discovered experimentally by Galileo in the early 17th century, who performed experiments with balls rolling on inclined planes. He also later proved this mathematically in his book Dialogue Concerning Two New Sciences. 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 forms 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 does not resemble a parabola.
Another hypothetical situation in which parabolae 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 close to being parabolic.
Approximations of parabolae 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. 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, e.g. bending, forces. 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 legend of debatable veracity, 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.
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.
Click on any image to enlarge it.
A bouncing ball captured with a stroboscopic flash at 25 images per second. Note that the ball becomes significantly non-spherical after each bounce, especially after the first. That, along with spin and air resistance, causes the curve swept out to deviate slightly from the expected perfect parabola.
The path (in red) of Comet Kohoutek as it passed through the inner solar system, showing its nearly parabolic shape. The blue orbit is the Earth's
Parabolic shape formed by a liquid surface under rotation. Two liquids of different densities completely fill a narrow space between two sheets of transparent plastic. The gap between the sheets is closed at the bottom, sides and top. The whole assembly is rotating around a vertical axis passing through the centre. (See Rotating furnace)
Parabolic microphone with optically transparent plastic reflector, used to overhear referee conversations at an American college football game.
Edison's searchlight, mounted on a cart. The light had a parabolic reflector.
Physicist Stephen Hawking in an aircraft flying a parabolic trajectory to produce zero-gravity
In algebraic geometry, the parabola is generalized by the rational normal curves, which have coordinates the standard parabola is the case and the case 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 (or other scalings), while the elliptic paraboloid is the graph of the positive-definite quadratic form (or scalings) and the hyperbolic paraboloid is the graph of the indefinite quadratic form Generalizations to more variables yield further such objects.
The curves for other values of p are traditionally referred to as the higher parabolas, and were originally treated implicitly, in the form for p and q both positive integers, in which form they are seen to be algebraic curves. These correspond to the explicit formula for a positive fractional power of x. Negative fractional powers correspond to the implicit equation 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 via algebraic geometry.
See also 
- Parabolic dome
- Parabolic reflector
- Parabolic partial differential equation
- Quadratic equation
- Quadratic function
- Rotating furnace, paraboloids produced by rotation
- Universal parabolic constant
- The only way to draw a straight line on the surface of a circular cone is to make it pass through the apex of the cone, where it will intersect the cone's axis. The line and the axis must therefore be coplanar.
- Apollonius' Derivation of the Parabola at Convergence
- 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
- Stargazer, p. 115.
- Stargazer, pp. 123 and 132
- Fitzpatrick, Richard (July 14, 2007), "Spherical Mirrors", Electromagnetism and Optics, lectures, University of Texas at Austin, Paraxial Optics, retrieved October 5, 2011.
- Lawrence, J. Dennis, A Catalog of Special Plane Curves, Dover Publ., 1972.
- In the diagram, the axis is not exactly vertical. This is the result of a technical problem that occurs when a 3-dimensional model is converted into a 2-dimensional image. Readers should imagine the cone rotated slightly clockwise, so the axis, AV, is vertical.
- Downs, J. W., Practical Conic Sections, Dover Publ., 2003.
- Dialogue Concerning Two New Sciences (1638) (The Motion of Projectiles: Theorem 1); see 
- 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.
- Troyano, Leonardo Fernández (2003). Bridge engineering: a global perspective. Thomas Telford. p. 536. ISBN 0-7277-3215-3., Chapter 8 page 536
- Drewry, Charles Stewart (1832). A memoir of suspension bridges. Oxford University. p. 159., Extract of page 159
- 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.
- Lockwood, E. H. (1961): A Book of Curves, Cambridge University Press
|Wikimedia Commons has media related to: Parabolas|
|Wikisource has the text of the 1911 Encyclopædia Britannica article Parabola.|
- Hazewinkel, Michiel, ed. (2001), "Parabola", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Weisstein, Eric W., "Parabola", MathWorld.
- Interactive parabola-drag focus, see axis of symmetry, directrix, standard and vertex forms
- Archimedes Triangle and Squaring of Parabola at cut-the-knot
- Two Tangents to Parabola at cut-the-knot
- Parabola As Envelope of Straight Lines at cut-the-knot
- Parabolic Mirror at cut-the-knot
- Three Parabola Tangents at cut-the-knot
- Module for the Tangent Parabola
- Focal Properties of Parabola at cut-the-knot
- Parabola As Envelope II at cut-the-knot
- The similarity of parabola at Dynamic Geometry Sketches, interactive dynamic geometry sketch.
- a method of drawing a parabola with string and tacks