← Ray Model of Light

Lesson 1 of 8 · Textbook 5.1

Rays, beams and ray diagrams

One straight line with an arrow on it. That is the entire model — and it is enough to explain everything else in the chapter.

Rays and beams

Shine a beam of light in the dark and watch what it does. Light travels along straight line paths, and each of those paths is called a ray.

Two words that are often muddled A ray is a single straight-line path taken by light.
A beam is a bundle of light rays — like the green laser beams in a light show at Marina Bay Sands.

We use the ray model of light to help us represent the path taken by light.

Proving light travels in straight lines

A candle and three cards, each with one hole, are placed in a black box. When all three holes line up, light travels from the flame into your eye and you see it. Move one card out of position and the light is blocked — you cannot see the flame at all.

Slide the middle card and watch what happens.

The three-card experiment

eye candle blocked

If light could bend, it would curve around the displaced card and you would still see the flame. It cannot — so it does not.

Drawing a ray diagram

Because light behaves this way, we can represent it with a straight line in a ray diagram. The line shows the path taken by the light, and the arrow shows the direction of travel.

What a model is for The ray diagram is an example of a model — a scientific representation of something. A model can be used to explain and predict our observations of how light behaves. It is not a photograph of light; it is a tool for reasoning about it.
How fast is it going? In a vacuum — space containing no matter — light travels 300 million metres in one second. At that speed you would go round the Earth 7.5 times in one second.

For comparison, signals in your brain travel at about 120 metres per second. An AI recognition system that identifies objects at the speed of light is therefore working millions of times faster than the nerve signals doing the same job in your head.

Check yourself