P6: Refraction of light

• To use ray tracing to measure the angles of incidence and refraction when light is refracted by a glass block.

• To demonstrate understanding that the angles of incidence and refraction are measured from a line at right angles to the glass surface known as the normal.

• And to use the measurements taken to plot a graph of angle of incidence against angle of refraction to show that they are related but not proportional.

There are different ways to investigate refraction in rectangular blocks.

In this required practical activity, it is important to:

• make, measure and record the angles of incidence accurately using a protractor.

• observe and use a protractor to measure angles of refraction.

What are the variables?

In this experiment:

Independent variable is the angle of incidence.

Dependent variable is the angle of refraction.

Control variables are the material of the block, the shape of the block and the colour of the light.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the angle of incidence.

What is the prediction?

Light is travelling from air to glass and so is refracted towards the normal.

However, as the angle of incidence increases the refracted light will bend from a bigger initial angle, and so the angle of refraction will also be bigger.

What apparatus is used in this practical?

Low voltage power pack, a 12V ray box, a single slit comb, a rectangular glass block, a sheet of white paper, a protractor, a sharp pencil.

Possible error

The main cause of error in this experiment is the measurement of the angles of incidence and refraction.

This can be kept to a minimum by:

• replacing the block carefully on its outline.

• ensuring that the power pack is set to 12 V, so that the ray box is at maximum brightness.

• doing the experiment in a dark room so that the emergent ray can be easily seen and marked.

Results

Graph

Plot a graph of angle of incidence, i in ° on the y-axis against angle of refraction, r in ° on the x-axis and draw the line of best fit.

Conclusion

We can see from the graph that as the angle of incidence, i, increases, the angle of refraction, r, also increases.

This agrees with our prediction.

However, the angle of incidence is not directly proportional to the and angle of refraction as the line of best fit is not a straight line through the origin.