# Representing Refraction With Ray Diagrams

## Representing Refraction With Ray Diagrams

It is useful to draw ray diagrams to understand how the geometrical optics concepts we have discussed previously work. Before we can draw the diagrams we need to define a few concepts such as the normal to a surface, the angle of incidence and the angle of refraction.

### Definition: Normal

The normal to a surface is the line which is perpendicular to the plane of the surface.

### Definition: Angle of incidence

The angle of incidence is the angle defined between the normal to a surface and the incoming (incident) light ray.

### Definition: Angle of refraction

The angle of refraction is the angle defined between the normal to a surface and the refracted light ray.

The diagram shows the boundary between two media: water and air. An incoming light ray is refracted when it passes through the surface of the water into the air. The angle of incidence is $$\theta_{1}$$ and the angle of refraction is $$\theta_{2}$$.

When light travels from one medium to another, it is refracted. If the angle of incidence is not equal to zero, the light ray will change direction from its original path as it is refracted.

Light is moving from an optically dense medium to an optically less dense medium.

Light is moving from an optically less dense medium to an optically denser medium.

## Optional Experiment: Propagation of light from air into glass and back into air

### Aim

To investigate propagation of light from air into glass and back into air

### Apparatus

ray box, rectangular glass block, plain paper, pencil, ruler, protractor.

### Method

1. Place the glass block on a plain piece of paper and use your pencil to draw around the block to make its outline on the paper.

2. Now turn on the ray box and aim the light ray through the left side of the glass block as illustrated in the diagram:

3. Now use your pencil to draw a dot somewhere on the incident light ray and another dot at the point where it enters the glass block.

4. Use your pencil to draw a dot at the point where the light exits the glass block and also somewhere else along the exiting ray.

5. Turn off the ray box and remove the glass block from the paper. Use your ruler to join the dots so that you have drawn a picture that looks like the figure above.

6. Now draw the normals to the surfaces where the light ray enters and leaves the block and mark the angle of incidence and angle of refraction on the left surface and the right surface.

### Questions for discussion

1. At the surface where the light enters the glass block, what do you notice about the angle of incidence compared to the angle of refraction?

2. Now look at the surface where the light exits the glass block. Compare the angle of incidence and angle of refraction here.

3. How do the optical densities and indices of refraction for air and glass compare?

A recommended experiment for informal assessment is also included. This investigates the propagation of light from one medium into another. Learners will need glass blocks of various shapes, a transparent container, water, plain paper, a pencil, a ruler, a protractor and a ray box. The questions for discussion at the end of the experiment get learners thinking about what they found in the experiment.

## Optional Experiment: Propagation of light from one medium into another medium

### Aim

To investigate the propagation of light from one medium into another

### Apparatus

ray box, glass blocks of various shapes, a transparent container filled with water, plain paper, pencil, ruler, protractor.

### Method

Starting with the rectangular glass block, repeat the steps below for each of the various differently shaped glass blocks:

1. Place the glass block on a plain piece of paper and use your pencil to draw around the block to make its outline on the paper.

2. Turn on the ray box and aim the light ray through one of the block’s surfaces.

3. Draw a dot at the point where the light enters the block and another dot somewhere else along the incident ray. Also draw a dot at the point where the ray exits the surface of the block and another somewhere along the exiting ray.

4. Remove the glass block and turn off the ray box. Use your ruler to join the dots.

5. Now draw the normals to the surfaces where the light ray enters and exits the block.

6. Use your protractor to measure the angles of incidence and refraction at the surfaces where the light ray enters and exits the block.

Now follow the same steps as before but place differently coloured filters at the surface of the blocks where the light enters the block.

Lastly, replace the glass blocks with the container of water and repeat the same steps as above. For this case, try to aim the ray box so that the angle of incidence on the water container is the same as it was for the rectangular glass block you investigated first. This is easier if you place your new piece of paper on top of the drawing you made with the rectangular block. Line up the water container where you drew the edge of your glass block. Then you can aim the ray box so that the light lines up with the incident ray on your paper and follow the steps as before.

### Questions for discussion

1. For each of your differently shaped glass blocks, how does the angle of incidence going from air to glass compare to the angle of refraction?

2. What happens when you place a coloured filter at the boundary between the air and a glass block?

3. When you compare your two diagrams for the rectangular glass block and the water container, if you did it correctly, the angles of incidence from air to glass/water should be the same. What can you say about the angles of refraction for glass and water respectively? How does this compare with what you already know about the refractive indices or optical densities of these materials?

### Optional Video: Light Bending PhET Simulation

See the video below describing light bending using PhET simulation.