Rectilinear propagation: Difference between revisions

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Rectilinear propagation describes the tendency of electromagnetic waves (light) to travel in a straight line. Light does not deviate when travelling through a homogeneous medium, which has the same refractive index throughout; otherwise, light experiences refraction. Even though a wave front may be bent, (e.g. the waves created by a rock hitting a pond) the individual rays are moving in straight lines. Rectilinear propagation was discovered by Pierre de Fermat.^[1]

Rectilinear propagation is only an approximation.^{[citation needed]} The rectilinear approximation is only valid for short distances, in reality light is a wave and have a tendency to spread out over time. The distances for which the approximation is valid depends on the wavelength and the setting being considered. For everyday usages, it remains valid as long as the refractive index in the medium is constant.

The more general theory for how light behaves is described by Maxwell's equations.

Proof

Take three cardboard A, B and C, of the same size. Make a pin hole at the centre of each of three cardboard. Place the cardboard in the upright position, such that the holes in A, B and C are in the same straight line, in the order. Place a luminous source like a candle near the cardboard A and look through the hole in the cardboard C. We can see the candle flame. This implies that light rays travel along a straight line ABC, and hence, candle flame is visible. When one of the cardboard is slightly displaced, candle light would not be visible. It means that the light emitted by the candle is unable to bend and reach observers eye. This proves that light travels along a straight path, as well proving the rectilinear propagation of light.

References

^ "Fermat's principle". Oxford Reference. Retrieved 2024-01-01.

This electromagnetism-related article is a stub. You can help Wikipedia by expanding it.

[1] "Fermat's principle". Oxford Reference. Retrieved 2024-01-01.

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-{{unreferenced|date=December 2016}}
+{{more citations needed|date=January 2024}}
-{{original research|date=September 2016}}
-'''Rectilinear propagation''' describes the tendency of [[Electromagnetic waves]] (light) to travel in a straight line. Light only deviates from a straight line when the medium it is travelling through changes density. This is called [[refraction]]. Light does not deviate when travelling through a homogeneous medium, which has the same [[refractive index]] throughout.
+'''Rectilinear propagation''' describes the tendency of [[electromagnetic waves]] (light) to travel in a straight line. Light does not deviate when travelling through a homogeneous medium, which has the same [[refractive index]] throughout; otherwise, light experiences ''[[refraction]]''. Even though a [[wave front]] may be bent, (e.g. the [[Surface wave|waves]] created by a rock hitting a pond) the individual [[Ray (optics)|rays]] are moving in straight lines. Rectilinear propagation was discovered by [[Pierre de Fermat]].<ref>{{Cite web |title=Fermat's principle |url=https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095815400 |access-date=2024-01-01 |website=Oxford Reference |language=en }}</ref>
-Even though a [[wave front]] may be bent, (e.g. the [[Surface wave|waves]] created by a rock hitting a pond) the individual waves are moving in straight lines.  With the sense of the scattering of waves by an inhomogeneous medium. An experiment can be set up to prove this. Three cardboard squares are aligned with a small hole in the center of each. A light is set up behind the cardboard. The light appears through all three holes from the other side. The light is blocked if any one of the cardboard squares are moved even a tiny bit. This proves that waves travel in straight lines and this helps to explain how humans see things, among other uses. It has a number of applications in real life as well. The types of rectilinear propagation of light are:
-* Pin hole camera
-* Formation of shadow
-* Eclipses
+Rectilinear propagation is only an ''approximation''.{{cn|date=October 2024}} The rectilinear approximation is only valid for short distances, in reality light is a wave and have a tendency to spread out over time. The distances for which the approximation is valid depends on the wavelength and the setting being considered. For everyday usages, it remains valid as long as the refractive index in the medium is constant.
-The farther the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered [[Proportionality (mathematics)|proportional]]). Also, if the object is moving, the shadow cast by the object will project an image with dimensions (length) expanding proportionally faster than the object's own rate of movement. The increase of size and movement is also true if the distance between the object of interference and the light source are closer. This, however, does not mean the shadow may move faster than light, even when projected at vast distances, such as [[light year]]s. The loss of light, which projects the shadow, will move towards the surface of projection at [[light speed]].
+The more general theory for how light behaves is described by [[Maxwell's equations]].
+== Proof ==
+Take three cardboard A, B and C, of the same size. Make a pin hole at the centre of each of three cardboard. Place the cardboard in the upright position, such that the holes in A, B and C are in the same straight line, in the order. Place a luminous source like a candle near the cardboard A and look through the hole in the cardboard C. We can see the candle flame. This implies that light rays travel along a straight line ABC, and hence, candle flame is visible. When one of the cardboard is slightly displaced, candle light would not be visible. It means that the light emitted by the candle is unable to bend and reach observers eye. This proves that light travels along a straight path, as well proving the rectilinear propagation of light.
-Although the edge of a shadow appears to "move" along a wall, in actuality the increase of a shadow's length is part of a new projection which propagates at the speed of light from the object of interference. Since there is no actual communication between points in a shadow (except for reflection or interference of light, at the speed of light), a shadow that projects over a surface of large distances (light years) [[Special relativity#Causality and prohibition of motion faster than light|cannot convey information]] between those distances with the shadow's edge.<ref>Philip Gibbs (1997) [http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3 Is Faster-Than-Light Travel or Communication Possible?] {{webarchive|url=https://www.webcitation.org/5lLRguF0I?url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html |date=2009-11-17 }}</ref>
 ==See also==
+*[[Diffraction]]
 *[[Plane wave]]
+==References==
+{{Reflist}}
 [[Category:Waves]]
-{{physics-stub}}
+{{electromagnetism-stub}}