Practical: Refractive index


In this practical the refractive index of a glass (or perspex depending on what you choose) block will be found by measuring the angles of incidence and refraction for a ray of light passing through it.


According to Snell's law, for a ray of light passing from air into glass the ratio fraction numerator sin space i over denominator sin space r end fraction is a constant equal to the refractive index of the boundary. Since the refractive index of air is approximately 1 we can take this to be the absolute refractive index of glass.


Place the glass block on a sheet of white paper and arrange the ray lamp so that a single ray of light passes through the block. Either mark the position of the rays and block by drawing on the paper or take a photograph of the ray with your phone / camera. Repeat the procedure with at least 5 different angles of incidence.

  • If you are using the pencil method then be careful to label the rays so you know which ray coming out of the block corresponds to which incident ray.
  • If using the photographic method then you can measure the angles using the on screen protractor that can be downloaded from here. This is very easy to use, click the place that you want to be the centre of you measurement then use the mouse wheel to rotate the axis.

To find the refractive index of glass calculate the values of fraction numerator sin space i over denominator sin space r end fraction for each angle and find the average value. Alternatively you could plot a graph of sin i vs sin r, the gradient will give the refractive index.

If you don't have enough apparatus to go round (I don't) here are some of my photos.

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