How to determine the refractive index of a glass block

Written by william hirsch
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How to determine the refractive index of a glass block
Light refracts through this lens to your eye. (Military stamp image by Yuriy Rozanov from Fotolia.com)

When a beam of light enters a piece of glass its path deviates from a straight line. This phenomenon is known as refraction. The behaviour of light travelling between one medium and another is governed by Snell's Law. Snell's Law states that the amount the light deviates from its original path depends on the incident angle of the incoming light and the index of refraction of the initial and final mediums. The amount the light bends physically is due to the speed of light being slower in the medium and is related to the materials index of refraction.

Skill level:
Easy

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Things you need

  • Laser
  • Glass block
  • Protractor
  • Scientific calculator
  • Pencil
  • Paper

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Instructions

  1. 1

    Aim the laser at the glass block so the beam is not perpendicular to the surface of the block.

  2. 2

    Measure the incident angle with a protractor of the laser beam. This angle is measured from an imaginary line perpendicular to the surface of the block that the beam entered. As an example, assume the angle is 30 degrees.

  3. 3

    Measure the refraction angle the beam makes inside the glass. The angle is measured from an imaginary line perpendicular to the inside surface of the glass that the beam penetrated. For the example, say this angle is 20 degrees.

  4. 4

    Solve Snell's Law for the index of refraction of the glass ("n2"). This leads to:

    n2 = n1 x sin (Incident angle) / sin (Refracted angle),

    where n1 is the index of refraction for the first medium which is air. The value of n1 is 1.00029.

  5. 5

    Substitute the values of the variables to find n2. This gives the result:

    n2 = ( 1.00029 ) x sin ( 30 ) / sin ( 20 )

     =  1.46.
    

    The index of refraction fro glass form the calculation is 1.46.

  6. 6

    Compare the result your calculate to the actual index of refraction for glass. Standard glass, otherwise know as crown glass, has an index of refraction of 1.52.

Tips and warnings

  • Make sure your calculator is in "degree mode" when calculating the sine of angles in degrees.

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