Properties of Light Rays

Light travels in Straight Lines

If light is travelling in a vacuum or a medium of constant refractive index, it travels in straight lines.

Light is Reversible

The if the light travels through different materials, it travels along a certain path which may include travelling through different media, in which it is refracted. If the path of the light is reversed then it will travel back along the same path.

This is known as the principle of reversibility and is an important rule in optics