Optics

Rectilinear propagation and reflection

In a transparent, uniform, homogeneous medium, light travels in straight lines. At a smooth mirror, a ray reflects according to a definite law.

In a transparent, uniform, homogeneous medium, light travels in straight lines. At a smooth mirror, a ray reflects according to a definite law.

Applications and calculation

Quick check

Test straight-line propagation by looking at a candle through three cards with small holes: the flame is visible only when the holes line up. A sharp shadow also shows that light travels nearly straight when an obstacle blocks it. A ray is a model for the direction of propagation, not a material thread; a beam contains many rays. In air, light is effectively straight over classroom distances, but its path bends when the refractive index changes, for example across a temperature gradient.White light can also split into colours in a prism, showing that the ray model must be supplemented by a wave model when wavelength-dependent effects matter.Shadow size also depends on the distance between the object and screen.

An extended source makes a penumbra as well as a shadow, so shadow edges need not be perfectly sharp.

At a smooth surface, reflection is orderly: the incident ray, reflected ray, and normal lie in one plane, and the reflection angle equals the incidence angle. On a rough surface, each tiny patch still obeys this law, but its normal changes from place to place, spreading reflected light in many directions. This diffuse reflection is why a wall can be seen from many positions. The light is redirected rather than simply destroyed, although some energy may be absorbed by the material.