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The prism spectrometer

Equipment provided for the session

Equipment provided for the session

Prism Spectrometer

In this session you will have:

Gained a proficiency in the reading of vernier scales


Learnt a method for the measurement of refractive index


Understood the accuracy and precision limits to which refractive index may be measured


Kept, and submitted, a clearly laid out set of laboratory notes


Accurate measurement of the refractive index of materials can be obtained using a prism made out of the material of interest. Rays passing though the prism deviate because of refraction. Simple ray (geometrical) optics and the use of Snell's law show that the index is directly related to the minimum angular deviation of a ray that passes through the prism at different angles of incidence. You can measure this minimum deviation directly for any ray that comprises a single wavelength of light (a monochromatic beam).

A cadmium lamp provides a few discrete wavelengths and for each wavelength the minimum deviation angle will be different. This is because the prism refractive index varies with wavelength (the property known as dispersion) and so you will also be able to measure this property as well.

There is an ISE where you can practice this experiment before your session. It can be found here.


Other Resources


The script for this experiment can be found in the lab script book or on DUO.