Travelling light(Exposition)

Light travels

The idea that light travels is not uncommon. Children will often refer to things moving at the speed of light when they are talking about things moving very quickly. In the simple models set out in the previous section, the light travels from a luminous source to the eye, the light travels from the torch to the object and then to the eye.

Light travels at:

300 million metres in each second or: 3 × 108 metre / second

To be precise, what we usually call the speed of light is really the speed of light in a vacuum. In reality, the speed of light depends on the material (often called a medium) that it moves through. Light moves slower in water and glass than in air, and in all cases the speed is less than in a vacuum.

Here are a few values of the speed of light in different media for your interest: vacuum, 299 792 000 metre/second ; air, 299 703 000 metre/second ; water, 225 408 000 metre/second ; glass, 199 862 000 metre/second.

The fact that light travels so quickly means that all of those day-to-day events involving light (such as light being reflected from the face of your watch and travelling to your eyes) appear to happen instantaneously, and of course to all intents and purposes they do. These experiences can undermine the basic idea of light as being something which is moving.

Teacher Tip: As we see, information travels from object to eye.