Aim : Determination of refractive index of a material of a prism.
Apparatus: A spectrometer, a spirit level, a source of monochromatic light (sodium
vapour lamp), a glass prism, a magnifying lens and a reading lamp.
Theory: When a beam of light strikes on the surface of transparent material (glass,
water, quartz crystal, etc.), the portion of the light is transmitted and and other portion
is reflected. The transmitted light ray has small deviation of the path from the incident
angle. This is called refraction.
Refraction is due to the change in speed of light while passing through the medium.
Formula:
It is given by snell’s law
n1 sin i = n2 sin r
here i is the angle of incident, r is the angle of refraction, n1 is the refractive index of
the first medium and n2 is the refractive index of the second medium.
When a ray of light passes through a prism, it suffers refraction as shown in fig. 1.
Diagram:
Procedure:
Determine the least count of the spectrometer.
2. Place the prism on the prism table with its refracting angle A towards the collimator
and with its refracting edge A at the centre. In this case some of the light falling
on each face will be reflected and can be received with the help of the telescope (as
shown in fig. 3).
3. The telescope is moved to one side to receive the light reflected from the face AB
and the cross wires are focused on the image of the slit. The readings of the two
verniers are taken.
4. The telescope is moved in other side to receive the light reflected from the face AC
and again the cross wires are focused on the image of the slit. The readings of the
two verniers are taken.
5. The angle through which the telescope is moved; or the difference in the two posi-
tions gives twice of the refracting angle A of the prism. Therefore half of this angle
gives the refracting angle of the prism.6. Angle of prism is calculated and found to be calculated as 60°.
Result : Refractive index of the material of the prism : .1.55
Precautions:
1. The telescope and collimator should be individually set for parallel rays.
2. Slit should be as narrow as possible.
3. Both verniers should be read.
4. The prism should be properly placed on the prism table for the measurement of
angle of the prism as well as for the angle of minimum deviation.
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