n
Prediction: I predict that when the light enters the Perspex block (which is optically more dense) it will bend towards the normal.
Therefore the angle of incidence will always be larger than the angle of refraction. Using Snell’s Law, as long as I know the angle of incidence and the refractive index, I can predict the angle of refraction. Using this equation:
R= sin sin i N = Refractive Index
n
Using this equation I am going to predict the angle of refraction for each of my 8 angles. The predictions are:
Angle of Incidence Equations Predicted angles of r
10 sin (sin 10 1.5) 6.6
20 sin (sin 20 1.5) 13.2
30 sin (sin 30 1.5) 19.5
40 sin (sin 40 1.5) 25.4
50 sin (sin 50 1.5) 30.7
60 sin (sin 60 1.5) 35.3
70 sin (sin 70 1.5) 38.8
80 sin (sin 80 1.5) 41
Fair Testing: To make sure the investigation is fair I plan to keep the following constant:
The Perspex block, each block is different, some may have irregularities in them.
Same material, so the light will travel at the same speed for all tests, because if light travelled faster in a different material the light would not bend as much.
Thickness of the light, if the light ray was large it would make it more difficult to get the exact centre.
Shapes of the block, the rectangular prisms are easier to measure, whereas a curved prism is not as easy.
Same colour light, white light is made up of different waves, some travel slower than others. If you changed the colour of the light half way through the experiment the results would be very unreliable. E.g. at the start you used red light, but then switched to violet light during the experiment. Since red light travels faster than violet light, the refractive index of the red light would be greater than the violet light.
Keeping the block on its larger surface, so it is easier to measure the angle of refraction than it would be on its side.
Brightness of the light,
The ray and power boxes, at the same no. of amps the light might shine brighter from one ray box than another. The power box may not give the correct amount of energy for a certain number of amps, i.e. too much or too little energy.
The only variable will be angle i. I have chosen eight angles, they are 10, 20, 30, 40, 50, 60, 70 and 80. I have chosen these eight angles because they give me a wide enough range to work with, and they are all equal distances apart.
I will repeat the experiment three times to get a mean, this also enables me to get more accurate and reliable results.
Safety: This experiment is safe. Although the light bulb contained in the ray box may become hot to the touch.
Experiment Design:
The equipment needed for this experiment is:
1 Ray box + singleslit plate
2 Power box
3 Rectangular Prism (Perspex)
4 Ready prepared sheets (see below)
Step one: Choose between 5 to 10 angles of incidence, making sure that they have a wide enough range to easily compare results. E.g. not 21, 22, 23, etc. Then draw this following diagram, putting each angle on it’s own page (preferably).
Angle i is the variable, you put the different angles of incidence at i.
Step Two: Plug the ray box into the power box putting red with red and black with black, then turn the electricity on. Set the power box on to 12 amps, or whatever brightness is required, just as long as the amps are kept the same throughout. Place the metal plate with the single slit pointing downwards into the ray box. As shown below:
Step Three: One at a time place the sheets with diagrams on in front of the ray box. Place the rectangular prism on the horizontal line, then draw another line beneath the prism, this will be where the ray of light exits the prism. Position the ray box/ sheet so that the ray of light travels down the centre of the angle i line, making sure it his the block at the normal.
Step Four: A ray of light will be refracted through the glass prism and will come out at the opposite side of where it was first directed. Draw crosses along the centre of the exiting ray of light.
Step Five: Draw a line through the centre of the crosses, until it hits the bottom line. Connect the point where angle i touches the normal to the exit line of light. The angle nearest the normal is the angle of refraction. Measure angle r for each diagram. To ensure an accurate and reliable experiment, repeat the whole experiment twice more.
Obtaining Evidence: I have repeated the experiment three times in order to gain reliable and accurate results. When measuring the angle of refraction I measured it to the nearest 0.5 of a degree.
Conclusion:
The average of refractive indexes is 1.45.
Apart from the 80, the results are all within 2 degrees.
I found that there is a rule that governs the angle light is refracted through. Angle of incidence has always been larger than the angle of refraction. Once I had finished the experiment I was able to measure the angle of refraction. Then using Snell’s law I was able to work out the refractive index. I used the following equation:
Refractive Index = Sine i
Sine r
All of my refractive index results came within 0.08 of 1.5. All of my observed angles of refraction are larger than predicted this may be due to experimental error, I may have measured an angle incorrectly. I could have made a wrong predication, or I used an incorrect refractive index for all of my predicted angles of refraction. The average of all my refractive indexes is 1.45, so the correct refractive index of Perspex will probably be between 1.45 and 1.5.
The graph, which plots the angle of incidence against the angle of refraction, shows a curve. This means that angle i increases more quickly than angle r. Showing that the angle of incidence will always be larger than the angle of refraction.
The graph, which plots the sine of incidence against the sine of refraction, shows a straight line. This means that they both increase at the same rate. The gradient of the line should be the refractive index, 1.5, which the gradient of the line is.
The graph, which plots the refractive index against the angle of incidence, is almost a straight line. The line is below 1.5, which shows that the refractive index of Perspex at 1.5 is incorrect. But the line is straight so I can say that the refractive index of Perspex remains constant.
My results support my prediction that the angle of incidence will always be larger than the angle of refraction. This is because when light enters the Perspex block which is optically more dense than air, it will bend towards the normal, since it is being slowed down. This can be seen from the graph, which has the angle of incidence against the angle of refraction. The angle of incidence increases more than the angle of refraction. Showing that angle i will always be larger than angle r.
Evaluation: My investigation is good, because it was safe and fairly accurate and reliable. The accuracy of my measurements is reliable, but I have identified two anomalous results. The tables in my conclusion show that for angles 10 and 80 the refractive index is 1.42. For the 10 degrees this mat be because it is quite hard to measure the angle accurately since it is so small. So an error in measuring on my part may be to blame. Experimental error could also have a part e.g. the light did not go down the centre of the line, the light may have entered the block at an incorrect angle. The 80 angle is also hard to measure since it is so near the 90. When the light entered the block it may have hit the opposite corner, which would scatter the light, making it hard to measure. It may have also been experimental (explained above) or a measuring error on my part. To obtain more reliable results I could repeat the two angles.
Although some of my results aren’t as reliable as the rest I believe that I have enough evidence to support a conclusion, that the rule which governs the angle that light is refracted through is Snell’s law.
To gain more accurate results all round I could repeat the whole experiment up to five times, which would give me a more reliable mean. Instead of using a ray of light, which spreads easily, I could use a laser, which is guaranteed not to spread. To measure the angle of r more accurately I could use a microscope, making the measurement more precise. If I carried the lines on I could use a larger protractor which would have accuracy up to 0.1 of a degree.
To extend the experiment on further I could use different materials for my prisms, e.g. diamond or glass. So I could then see the differences in the angle of refraction, whether it is smaller or larger. I could change the shape of the prism, e.g. rounded or triangular, but that would also make it harder to measure the angles since the light would be dispersed in a different way. Could use different coloured lights, violet light and red light are at the opposite ends of the spectrum, the red light should have a larger angle of refraction since it travels faster than the violet light.
Bibliography: ‘Physics for You’ by Keith Johnson, I obtained Snell’s law from this book and information on light and refraction.
By Cheryl Gogin
