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The aim of this experiment is to investigate whether the colour of light incident on a medium affects its refractive index.

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Refractive Index

Praveen Ravi

G 11 Lab Report

Aim – The aim of this experiment is to investigate whether the colour of light incident on a medium affects its refractive index.

Background – Refraction is the bending of light when it passes from one medium to another. Refraction occurs because of the change in density in the new medium which changes the amount of obstruction of the light causing the light to deviate from its original path and take a new, shortest one through the new medium. Refractive index is a unique property of transparent and translucent materials. It is governed by Snell’s law μ = Sin i / Sin r where i and r are the angles of incidence and refraction respectively and μ is the refractive index and is defined as “A property of a material that determines how fast light travels through it.”[1]1

Hypothesis – I believe that the refractive index of a transparent or translucent medium is independent of the colour of light incident on it. Light always travels in a straight line. When a ray of light enters a medium at a certain angle, it is forced to bend because of the change in density in the new medium and thus, a change in obstruction.

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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°

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                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]1 Intel, Silicon Photonics Glossary – Glossary of terms relating to Silicon Photonics.http://www.intel.com/technology/silicon/sp/glossary.htm (updated 10th April 2005, accessed 10th April 2005)

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