Light, Sound and Waves

Travelling light

Physics Narrative for 5-11 11-14 Supporting Physics Teaching

Light travels

The idea that light travels is not uncommon in the 21st century. Pupils 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.

Teacher: Just how fast does light travel?

The answer is 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 more slowly 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-1 ; air, 299 703 000 metre second-1 ; water, 225 408 000 metre second-1 ; glass, 199 862 000 metre second-1.

We'll return to this slowing down of light as it passes into different substances (such as water or glass), when we consider the refraction of light in episode 03.

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 essential idea that light necessarily involves movement.

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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