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Refractive Index of Prism Experiment (Class 12)

 

The Refractive Index of Prism Experiment using Spectrometer

Aim: To determine refractive index of the material of the prism using spectrometer.

 

Apparatus: Spectrometer, Prism, Sodium vapour lamp etc.

 

Principle:

  

The refractive index of the material of the prism,

Where, A - Angle of the prism

D - Angle of minimum deviation

 

Procedure:

 

i. Preliminary Adjustments

 

The telescope is turned towards a white wall and the eye piece is pushed in or out to get a clear image of the cross wire. The telescope is then focused to a distant object. The distance between the eye piece and objective is adjusted by means of the screw to get a clear image of the distant object. The rays from the distant object will be parallel through the telescope.

 

A monochromatic source of light like sodium vapour lamp is used to illuminate the slit. The telescope is brought in a line with the collimator and the source of the light so that an yellow patch of light is seen through it whose width may be adjusted by adjusting the width of the slit. The distance between the slit and the lens of the collimator is adjusted by means of the screw to get clear image of the slit. Leveling of the prism table can be done by a spirit level.

 

ii. To find the angle of the prism (A)

 

The least count of the spectrometer is found out. The prism is mounted on the prism table with its grounded surface towards the clamp and with its edge (A) turned towards the collimator (fig). The vernier table is clamped. The telescope is rotated till the image of the slit reflected from the face AB is obtained. The telescope is fixed using the radial screw. By working the tangential screw with vertical cross wire is made to coincide with the image. The readings on the main scale and verniers are recorded. The telescope is released and turned to observe the reflected image from the face AC. As before, the readings are recorded. The difference between these two readings on the same vernier is 2A. Thus the angle of prism (A) is calculated.

 

iii. To find the angle of minimum deviation (D)

 

The prism table is rotated so as to have the edge of the prism turned away from the collimator. Turning the telescope towards the base of the prism, the refracted image is observed. The vernier table is slowly rotated in the direction, which will reduce the angle of deviation, following the image with the telescope. At a particular position, the images is found to remain stationary for a moment and then begins to move in the opposite direction. The position at which the image just turns back is the minimum deviation position. The vernier table and telescope are fixed. The vertical cross wire is made to coincide with the image by adjusting the tangential screw of the telescope. The readings on the scale and the verniers are taken. The prism is removed and the telescope is brought in a line with the collimator. By adjusting the tangential screw the vertical cross wire is made to coincide with the direct image. The readings are again taken. The differences between these two readings give D, the angle of minimum deviation. 


Then the refractive index of the material of the prism is calculated using the formula,

Observations and Readings:

 

Magnitude of 1 m.s.d = _______

No: of divisions on the Vernier, N = _______

Least Count (L.C.) = 1 m.s.d/N = _______

 

i. To find the angle of prism (A)

 

Vernier I

Vernier II

MSR

VSR

Total = MSR + VSR x LC

MSR

VSR

Total = MSR + VSR x LC

degree

division

degree

degree

division

degree

Image reflected from face AB (i)

 

 

 

 

 

 

Image reflected from face AC (ii)

 

 

 

 

 

 

Difference b/w (i) and (ii). 2A

 

 

 

 

 

 

 

Mean 2A =____

A = ____

 

ii. To find the angle of minimum deviation (D)

 

Vernier I

Vernier II

MSR

VSR

Total = MSR + VSR x LC

MSR

VSR

Total = MSR + VSR x LC

degree

division

degree

degree

division

degree

Reading of refracted omage (i)

 

 

 

 

 

 

Direct reading (ii)

 

 

 

 

 

 

Difference b/w (i) and (ii). D

 

 

 

 

 

 

 

Mean D = ______ degree

n = _____


Result:

 

Refractive index of the material of the prism =   ________

 

Spectrometer Experiment Viva Questions and Answers

 

1. What is refraction of light?


The phenomenon of the bending of light from its straight path at the surface of two different optical media is called refraction of light.

 

2. What are the laws of refraction?


(a) The incident ray, the normal to the point of incidence and the refracted ray are in the same plane.

(b) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for any two given media. It is also called Snell’s law.

 

3. What is the unit of refractive index?


It has no unit since it is the ratio of two similar quantities.

 

4. What happens, when a prism is set in minimum deviation position?


(a) The angle of emergence becomes equal to the angle of incidence.

(b) The refracted ray travels parallel to the base of the prism.

 

5. What are the uses of a spectrometer?


To find the refractive index and dispersive power, to study spectrum, to find unknown wavelength etc.

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