University of Edinburgh

BSL Physics Glossary - ray of light - example

I'm going to talk about light and how it bends. You know light travels in straight lines. But sometimes light bends. We are going to look into why this happens. You might be able to do this yourself at home if you can find a window or thick glass with light going through. You can see light bending best if you use quite a large block such as glass or water. I'm going to show you what happens when light hits a glass block.

Before we start, this is the equipment you need for this experiment.

Right. Now take the glass block.  

(1st demo of light here)

Did you see how the light didn't go straight through the glass block? The light bends as it enters the glass block and again as it comes out. That's light bending. Some of the light is reflected where it hits the glass block, and some of it is refracted through the block. Now we are ready to mark the paths taken by the light.

(2nd demo of light here. Light paths are given little marks.)

Next I am going to draw 'normal' which is at 90 degrees to the side of the glass block from where the light hits the block.

(3rd demo with the protractor)

Next I am going to show the direction of the rays of light with arrows on the diagram. One ray is refracted and the other reflected.

(4th demo adding arrows to light beams)

I am going to use a protractor, to see if the angle of incidence is exactly the same as the angle of reflection.

(5th demo adding in angles)

You see how both angles are 47 degrees. These two angles are always the same as each other. The angle of incidence might be larger, and in that case the reflected angle will be larger too.

Now let's measure the angle of refraction.

(6th demo measuring angle of refraction)

It is 29 degrees. Not 47 degrees this time. The reason the light bends is that the materials it travels through are different. Air is less dense than glass. The ray of light meets the glass and slows down as it crosses the denser material. As it crosses back into the air it speeds up again. Look at the angle the light takes as it emerges from the block - angle of emergence.

(7th demo measuring angle of emergence)

The angle of emergence is 47 degrees. It is the same as the angle of incidence. The angle of incidence is equal to the angle of emergence. This is always true, providing that you are using a glass block with parallel sides. The incident ray will also be parallel to the emerging ray.

Now look at the incident ray.

Now look at the reflected ray. This is where part of the ray of light is reflected. It's a reflection from the block.

Refraction - part of the ray of light is refracted through the block.

And the light emerges here and continues in a straight line - emergent ray.

This is the angle of incidence and this is the angle of reflection.

So, the angle of incidence is equal to the reflected angle. And a ray also is refracted across the block of glass. This is the angle of refraction.

Here is the angle of emergence.

Now I'm going to turn the lights off and put the glass block back, to see if the reflected ray is correct on the diagram.

I am going to see if the light will follow the same path from a different direction. I am going to put the light off now.

Whichever way the light comes in, the angle of incidence and angle of reflection are the same. This is the principle of reversibility. It works both ways round.