Investigating refraction in glass prisms

In a nutshell

By using a ray box and glass block, you will investigate the relationship between the angle of incidence and angle of refraction to see how light is refracted. You will do this by varying the angle of incidence and measuring the angles of refraction in and out of the glass block.

Equipment list

The following equipment list can be used to conduct the experiment to investigate refraction in glass blocks.

Equipment	Use
Ray box and single slit grating	To create a narrow beam of light.
Glass block	To refract the light ray.
Paper	To show the direction of the light ray.
Pencil	To mark the direction of the light ray.
Ruler	To ensure the lines of the light ray are drawn straight.
Protractor	To measure the angles of the light ray.

Experimental variables

The independent variable is the variable you change. The dependent variable is the variable that changes depending on the other variables, and is the one you measure. The control variables are the variables that are kept constant.

Independent Variable	$Angle\ of\ incidence$
Dependent Variable	$Angle\ of\ refraction$
Control Variables	$Glass\ block,\ glass\ block\ placement,\ ray\ box$

Safety precautions

When performing experiments, it is very important to consider safety precautions. This is so that no one gets hurt or injured during the experiment.

Hazard	Risk	Safety Measure
Ray box	The box gets very hot and could result in a burn.	Do not touch during use or straight after use. Leave it to cool before putting it away. If burnt, run burns under cold water.
Light ray	Looking into the light may damage eyes.	Don't look directly at the light. Keep the ray box directed away from you.
Dark room	There may be trip hazards that are difficult to see due to the lack of light.	Take care when moving about, and try to limit movement. Check for hazards before dimming lights.

Method

These are the instructions to complete the experiment. Record all measurements taken as you work through the experiment in a table.

Physics; Physics practicals; KS4 Year 10; Investigating refraction in glass prisms

1.	Power supply
2.	Ray box
3.	Ruler
4.	Glass block
5.	Protractor
6.	Paper
7.	Refracted ray

1.	Set up the apparatus as shown in the diagram. The glass block should be placed on top of the sheet of paper.
2.	Draw pencil lines around the edge of the glass block. This means it can be placed back if anything goes wrong.
3.	Switch the ray box on and direct the light ray at the side of the block.
4.	Using the protractor, draw a line at $90\degree$ to the surface of the glass block, where the light ray hits it. This is the normal.
5.	Position the ray box so that it makes an angle with the normal (angle of incidence) of $20 \degree$ . Draw another normal at the point where the light ray exits the box. Note: $20 \degree$ is just an example angle. You can choose any angle you would like!
6.	Mark on the paper: A point on the ray close to the box. The point the ray hits the surface of the block (where you have drawn the normal). The point the ray exits the glass block. A point on the ray at a distance from the glass block.
7.	Remove the block and, using a ruler, join up these points to trace the outline of the light ray.
8.	Measure the angles of refraction and incidence using a protractor. You should measure three angles, the angle of refraction in the glass, the angle of incidence in the glass, and the angle of refraction exiting the glass. Note: The angle of incidence where the light hits the box should be equal, or close to, the angle of refraction for the light exiting the block.
9.	Replace the glass box and repeat steps 5-8 with two more angles of incidence. For example, $40 \degree$ and $60 \degree$ .

Analysis

This is how you will use the data recorded to form conclusions.

1.	Compare the angles of incidence and refraction for the two boundaries for each light ray.
2.	Compare these to known theory. The light ray slows down when entering an optically more dense medium (air to glass) so the angle of refraction should be smaller than the angle of incidence. The light ray speeds up when entering an optically less dense medium (glass to air) so the angle of refraction should be greater than the angle of incidence.
3.	Plot a graph of angle of refraction against angle of incidence. One line should join up the data points for air to glass, and another should join the points for glass to air.

Conclusion

Your results of your analysis should show that the measured experimental data matches with the theory of refraction. This means that the light ray bends away from the normal when it enters an optically less dense medium, and bends towards when it enters an optically more dense medium.

Physics; Physics practicals; KS4 Year 10; Investigating refraction in glass prisms

The results from the graph should show an approximately linear relationship between the angle of refraction and angle of reflection. The gradient of the glass to air line should be steeper than the air to glass line.

Note: The relationship between the two angles isn't actually linear! It just seems approximately linear when using these angles.

Evaluation

Once you have completed your experiment, it is important to consider how it could be improved. You should come up with ways to improve the accuracy of your values. If you got a result that seems unreasonable, think about why this was.