Rectilinear propagation

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.

Even though a wave front may be bent, (e.g. the 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

The farther the distance from the object blocking the light to the surface of projection, the larger the silhouette (they are considered 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 years. The loss of light, which projects the shadow, will move towards the surface of projection at light speed.

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) cannot convey information between those distances with the shadow's edge.[1]