Protractor x 1
Pencil
Ruler
Plain white paper
Variables - I will keep the angle of incidence constant.
I will change the colour of light incident on the Perspex.
I will measure the angle of refraction.
I will derive the refractive index of the Perspex.
Plan – The plan is to let a beam of white light from the ray box fall on the Perspex. It will be refracted. A normal will be drawn perpendicular to the point where the light hits the Perspex slab. Another normal will be drawn perpendicular to the point where the light comes out from the Perspex. The incidence path and the refracted path will be traced and the angle of incidence and refraction calculated. Then, different filters will be placed in front of the white beam and the same process will be carried out for different colours for the same angle of incidence. In each case, μ will be calculated.
Procedure –
- Draw the outline of the Perspex slab on the white paper.
- Tape the edges of the white paper to the table and the Perspex to the white paper to prevent both from moving.
- Connect the ray box to the power pack, A.C. 12V.
- Introduce a plastic piece with a very thin slit to allow a very thin beam of light in the slot in front of the ray box.
- Let the beam of white light be incident on the Perspex at 79°. This angle should be with respect to the normal and not with respect to the outline of the Perspex.
- Make a point where the beam hits the Perspex and trace the incident path.
- Make a point where the beam comes out of the prism and trace the path of the beam after coming out of the Perspex.
- Join both the dots to get the refracted ray.
- Measure the refracted angle and perform Snell’s equation to get a value of μ.
- Add light filters in front of the plastic piece to get thin beams of different colours.
- Repeat the procedure.
- Change the angle of incidence to 60°.
- Repeat the same process and calculate the value of μ.
Results – The results are as follows:
Trial 1
i = 75°
r = 79°
μ = Sin i / Sin r
= Sin 75° / Sin 79°
= 0.984
Trial 2
i = 60°
r = 61.5°
μ = Sin 60° / Sin 61.5°
= 0.986
Conclusion – It can be seen from the results that the values of the refractive index calculated in both trials are very close; there is a very small difference between them. It may mostly be because of an experimental error. It confirms the claim made in the hypothesis that the refractive index is independent of the colour of light incident on the medium.
However, the claim made in the hypothesis is wrong. Refractive index does depend on the colour of the light incident on the medium, mainly the wavelength. Scientific experiments have shown that colours of light with a shorter wavelength are refracted more than a colour with a longer wavelength, i.e. violet is bent the most and red is bent the least. This can be explained by saying that with a shorter wavelength, we have a greater frequency or more number of waves. When more number of waves hit the medium, the part that hits the wave first is refracted. Right after that, other waves follow up and force the waves before them to refract quickly. This rush that is caused can be imagined such that the waves want to refract quickly and thus push the waves preceding them with a large force one after the other making the preceding waves bend by relatively large angles. With waves of longer wavelength and lower frequency, the rush created will be much less. The number of waves hitting their preceding waves will be less because the waves are placed over large distances. This will cause the waves to bend at relatively smaller angles. Thus, for the same angle of incidence, different colours will bend at different angles. This can also be explained on the fact that if all colours bent at the same angles, then dispersion of white light would never have taken place and we would never have known that white light was a mixture of several different colours. Because each colour bends at different angles, white light’s components bend at different angles and give us a spectrum of different colours.
Evaluation – The conclusion suggests that the hypothesis made was incorrect. However, the results obtained support the claim made. This suggests that there must have been errors in the experiment.
One possible source of error is that the ray of light coming out of the ray box was not collimated despite the adjustment made. This causes problems because I assumed the ray from the slit card to hit the Perspex slab as parallel rays but they were not parallel. This means that each end was of a different angle. However, this difference between these angles would be very small because the width of the ray was very small and using small angle approximation, the Sinus of the two different angles would be very close to the value of the angles themselves and thus, negligible.
Another source of error could be the point chosen on the ray of light. Despite the small thickness of the ray of light, the light opened up or diffracted more at the point it hit the slab. The centre of the ray was chosen but there could be errors in judging the points. Since the same points were taken for every colour in each trial, this counts as a systematic error which can upset the results.
Other sources of error could be with the apparatus chosen. There could have been faults with the filters i.e. they might not be true red or true green for example. These differences will produce a light of a different wavelength that may appear to be a common colour like red or green but in reality, could be a shade of it. Again, this means that it is very difficult to produce a true colour. It could have affected the results because maybe with a true colour, different angles could have been obtained which would have proven the hypothesis wrong.
Measurement uncertainties were not taken into account, mainly with the protractor. This would certainly cause errors in the angles.
I would try to improve this experiment in many ways. First, I would include measurement uncertainties in my results to obtain a precise and accurate result. I would try to make the rays of light as collimated as possible i.e. to try to make them as parallel to each other as possible. This would ensure that there is negligible difference in the angles at the two ends of the light. I would also use a narrower slit to make the ray of light as thin as possible. This would ensure that choosing a point on the ray is not very difficult. I would also either bring the Perspex closer to the slab or the ray box closer to the Perspex to minimise diffraction. I would do the experiment for different colours in each trial, separately instead of just on one
This would make results more accurate by having separate results. I would also perform more number of trials to minimise random errors. I might probably use more accurate filters or rather lasers of different colours to have a monochromatic light than a polychromatic light since they are collimated and diffract less than polychromatic beam of light.
1 Intel, Silicon Photonics Glossary – Glossary of terms relating to Silicon Photonics. (updated 10th April 2005, accessed 10th April 2005)
