The rectilinear propagation of light

The light ray

The path of light is represented by a light ray: a straight line with an arrow indicating the direction of propagation, from the source to the illuminated object. A set of rays forms a light beam, which may be:

• parallel (the rays never cross, as with a very narrow laser beam),
• convergent (the rays come closer together and then meet at a single point),
• divergent (the rays spread out, as with the light from a torch).

A fundamental rule

In a transparent and homogeneous medium (such as air or a vacuum, with no obstacles), light always travels in a straight line. This is the principle of the rectilinear propagation of light.

A simple experiment

If you switch on a laser pointer in a room where there is a little smoke (or chalk dust), you can clearly see a straight line: this is visual proof that light travels in a straight line as long as no obstacle stops it or deflects its path.

Practical application: aiming

When you aim at a target with an imaginary arrow whilst closing one eye, you are already using this principle: your eye, the sight and the target must be aligned on the same straight line to aim properly.

Pitfall to avoid

Note: this principle only holds true in a homogeneous medium. If light changes medium (for example, from air to water), it can change direction: this is refraction, a different phenomenon which will be studied later. Also, do not confuse a light ray (an imaginary line) with a light beam (a collection of rays, visible in smoke or fog).