Refractive index by tracing light rays

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Physics Laboratory Report

Refractive index by tracing light rays

Date of experiment: 24/3/2009

Aim of experiment:

The objective of this experiment is:

  1. to find out the refractive index of a glass block by Snell’s Law
  2. to find out the refractive index of water by Snell’s Law

Theory:

In Part A of this experiment, the angle of incidence (i) and the angle of refraction (r) between air and a glass block were measured.

As stated by Snell’s Law, the following relation can be concluded:

nair sin i = nglass sin r

As nair = 1,                sin i / sin r = nglass

where nair and nglass are the refractive indexes of air and the glass block respectively.

Hence, by the above equation and measurements of i and r, the refractive index of the glass block can be determined.

In Part B of this experiment, the angle of incidence (i) between water and the glass block were measured.

As stated by Snell’s Law, the following relation can be concluded:

nglass sin i = nwater sin r

By substituting r =90o,           nwater = nglass sin i

where nwater is the refractive index of water.

Hence, by the above equation, as the refractive index of water is found and i is measured, the refractive index of water can be determined.

Apparatus:

Rectangular glass block                x 1

Pins                                                x 4

Drawing board                        x 1

Protractor                                        x 1

A4 paper                                        x 2

Beaker of water                        x 1

Procedures:

Part A: Refractive index of a glass block

  1. A sheet of paper was fixed on the drawing board and the glass block was placed in the middle. The outline of the glass block was sketched by using a sharp pencil.
  2. The glass block was removed and 5 straight lines were drawn to represent light rays with angle of incidence varying from 25o to 65o as shown in Fig.1. The lines were labelled as Ray 1 to Ray 5. For each line, the normal is drawn as dotted line and the angle of incidence (i) was measured.
  3. The glass block was placed back to the original position.
  4. Two drawing pins were placed vertically at any two points along Ray 1.
  5. The pins were viewed from the opposite side of the glass block and two other pins were placed so that the four pins appeared in line. The path of Ray 1 was completed.
  6. The angle of refraction (r) in the glass block was measured and the results were recorded in a table.
  7. Steps 3 to 6 were repeated with Rays 2 to 5.
  8. A graph of sin i against sin r was plotted.
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Part B: Refractive index of water by critical angle method

  1. A sheet of paper was fixed on the drawing board and the glass block was placed in the middle. The outline of the glass block was sketched by using a sharp pencil.
  2. A small strip of paper of about 1cm wide and of length equal to the thickness of the glass block was cut and a fine dark line was drawn down the middle of the strip. It was smeared with water and stuck on the longer side AD of the glass block. A vertical ...

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