The law of refraction (Snell’s law) deals with the sines of the angles involved. It is most simply stated mathematically: if the angle of incidence is called I and the angle of refraction is r, then sine i/sin r is a constant, n. The constant n is known as the refractive index of the material and is a measure of how much the light is refracted on passing through the surface.
Plan:
In this investigation, in order to find out the relationship between angles of incidence and angles of refraction for a transparent material and to find the critical angle of the transparent material, I am going to use a semi-circular block of Perspex and obtain at least six readings of values i and r. I will then plot a line graph of I against r and I will then use the graph to estimate the critical angle, this will be the value of the angle of incidence when the angle of refraction is 90 degrees.
Prediction:
I predict that the higher the angle of incidence will mean the higher the angle of refraction, so the high refractive index of a Perspex will give it a low critical angle of around 45 degrees
Every measurement of the angle of incidence will be around five to eight degrees and I will use a protractor to measure this
Method:
In this experiment, the only thing, which I am going to alter, is the distance of the ray box. I will move the ray box around five to eight degrees each time, so that I can achieve a range of different results.
Fair test:
In this experiment I am going to use the same block of Perspex each time I take a result because if I were to use a different block of Perspex each time, it may give out different results as to the block of Perspex which I am going to use. I will also place my block of Perspex on the same place on my piece of plain paper as an alteration could modify the results. I will be able to do this because I will draw around the block of Perspex the first time and from there on I will just place it in those guidelines.
Equipment:
- A block of Perspex
- A raybox
- A protractor
- Two wires
- Plain piece of A4 paper
- Plug socket
- Pencil
- Battery pack
Whilst conducting the experiment, the safety precautions, which I am going to take, are: I will place all the equipment in the middle of the table, so that any equipment does not fall and cause any nasty injuries to anyone. I will also make sure that I don’t switch on any electricity on until I had all the wires connected and was ready, so that there would be no risk of anyone having an electric shock.
Method:
- Collect all the equipment and place it in the middle of the desk.
- Connect wires to the battery pack and the raybox.
- Connect battery plug to the plug socket and switch the plug on.
- Place block of Perspex on plain piece of paper and draw around it.
- Move the raybox five to eight degrees each time using a protractor and draw in results with a pencil.
Once I had all my results I drew a table with my results filled in:
After I completed my table, I noticed that as my angle of incidence became greater so did my angle of refraction but this was by a greater margin than the angle of incidence.
I then drew a line graph of the angle of incidence against the angle of refraction. Once I had plotted all the points I matched them up by hand using a pencil and this gave me a smooth curve. However I did notice that the point (16,20) was a bit of track compared to the others so I did not add it in the curve. I think that the point (16,20) was a bit of track because I might of made a slight mistake when taking that reading of that point.
Using the graph (next page) that I drew, I found out that the critical angle for a block of Perspex is 54 degrees. I already know that the critical angle for glass is 42 degrees and I know that the critical angle for a block of Perspex is around that area as well so I know that I can’t be to far of.
I then drew out a table for sine i and sine r:
I worked out the values for sine i and sine r by pressing the sine button on my calculator followed by the value of either the angle of incidence or the angle of refraction. For example, when the angle of incidence was 10, I pressed the sin button on my calculator followed by 10 and that gave me the answer, in which in this case it was 0.1763 and I did the same for the angle of incidence. The final column of my table shows the answers of when sine i is divided by sine r.
Evaluation:
After completing this investigation I have realised that the relationship between the angles of incidence and the angles of refraction is that as the angles of incidence become greater so do the angles of refraction but the angles of refraction increase in a larger amount and the first table which I drew shows this. I also think that the critical angle for a glass block is around 54 degrees, it may not be exactly 54 degrees but I know that it is around 54 degrees.
In this investigation, I have learnt a lot about refraction, which I did not know before. I have learnt that refraction is always caused by the waves changing speed and that when waves slow down they bend towards the normal.
I think that most of my results are pretty accurate, except for one of the results, which is a bit of track and my line graph, shows this. I think that I carried out the experiment well. I think that if I were to carry out this investigation again then I would probably make sure that my results were more accurate by either making sure that I am moving the raybox the same distance each time or by carrying out the experiment twice and comparing the results. If I were to carry out the investigation again then I would complete the experiment much quicker and so I would save a lot of useful time.