Carry out an experiment to find a relationship between the incident angle and the refracted angle when light is shone through a rectangular Perspex block.

* A light slit.

* A rectangular Perspex block.

* A power pack.

* A protractor.

* A ruler.

* A pen or pencil.

* A sheet of paper.

Method

Firstly, I will draw around the Perspex block; this enables me to draw in the normal. I will also include the angles that I want to test, using the protractor. I will insert the light slit into the light box and shine the beam of light at the Perspex block, from the selected angles. Marking crosses at the point at which the light exits the Perspex and where it continues to travel. Then, when I connect the lines on the drawing I can measure the angle of refraction with the protractor.

Diagram showing the experiment

Prediction

When the light enters the Perspex block it will bend towards the normal. When the light exits the Perspex block it will return to its original angle. When the light is shone at the Perspex at an angle of 0

° There will not be a change to the direction of light, as each edge of the light slows down at the same point, keeping the light in a straight line.

Recorded results

By carrying out this experiment I have discovered that the light, when entering the denser medium bends towards the normal, making the angle of refraction smaller than the angle of incidence. The rays bend away from the normal as they emerge into the less dense medium.

When the light waves hit the boundary of the denser medium along the normal, which is exactly 90°, then there was no change in direction.

Below is a table showing my results.

Angle of incidence

Refracted angle

Mean of results

0°

0°

0°

30°

20°

25°

40°

23°

31.5°

45°

25°

35°

60°

32°

46°

According to the table above, the refracted angles are about half of the angles of incidence.

60° and 32°, 40° and 23° are examples of this.

0° doesn't change, because the different sides of the light beam slow down at the same time, which stops the light from bending. The refracted angle furthest away from the incident angle is 20°. This is 5° from what I would have expected.

I repeated this angle once more and obtained the following result.

.

30°

7°

23.5°

This final result seems to be more accurate when compared with the original result of 20°. The first result may have had some inaccuracies, as the light may not have been shone directly from 30°.

In this experiment, you can measure the amount of refraction that takes place in any boundary by measuring the refracted angle. To find the any given boundary we can compare the angle of refraction to the angle of incidence. The lights speed can make a large difference to the angle of refraction.

Analysis of results

Below is a graph to show the table of results.

The graphs line of best fit goes diagonally across the centre of the graph. This is because the refracted angle is half of the incident angle. From my graph I am able to find the refracted angle of any angle of incidence.

The results from my experiment support my prediction. As the light entered the Perspex block it bent towards the normal. And then when it exited the Perspex it returned to its original angle.

Conclusion

The results from my experiment could have been improved by decreasing the width of the beam of light from the light box. This would have made it easier to read the results since I would have been able to find the central point of the light beam with more ease. This may be why my results show that the refracted angles are just over or under half of the angle of incidence.

There are no particularly unusual results in my results experiment as they were all within three degrees of half of the angle of incidence. The accuracy of my results may have been improved if I had been more exact with my reading the angles and making sure the light beam was directed accurately at the selected angle.

The more that light refracts, the bigger the difference between the refracted angle and the angle of incidence.

As light travels into a denser medium it causes the wave length to change but the frequency of the wave never changes.

Refraction is always caused by waves changing speed. When the waves slow down they always bend towards the normal, when light entered the Perspex block it slowed down.

So I conclude that the refracted angle is half the angle of incidence.

Preliminary research

Preliminary experiment using semi-circular block

In addition to the experiment with the rectangular Perspex block, I performed another preliminary experiment using a semi-circular Perspex block. This had the same aim as the original experiment, as we were measuring the angles of refraction. The difference between these experiments was that the light was slightly reflected in the semi-circular block. This made reading the correct angle more difficult, as there were often several beams of light exiting from the block.

Equipment used

For this experiment I will need:

* A light box.

* A light slit.

* A semi-circular Perspex block.

* A power pack.

* A protractor.

* A ruler.

* A pen or pencil.

* A sheet of paper.

Method

I will draw around the Perspex block; this will then enable me to draw in the normal and the selected angles. I will shine the beam at the Perspex from the selected angles.

Marking crosses at the point the light exits the Perspex and where the light continues. Then, when I connect these lines I will be able to measure the angle of refraction with the protractor.

Table of Results

Angle of incidence

Refracted angle

0°

0°

0°

7°

20°

30°

30°

50°

40°

74°

50°

34°

60°

24°

Below is a graph showing my results for this experiment.

This graph is showing that as the angle of incidence increases, the refracted angle also increases. It follows a curved shape. The result found for 50° seems to be the only incorrect result; this could be because of the beam of light not being placed directly on the selected angle. The rest of the results seem accurate as they follow a predictable pattern. As in the wave theory, the curve of the graph follows a slight transverse wave pattern.

Joe Ramage 11H

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Investigating the refraction of light

Joe Ramage 11H