pHYSICS

Experiment 3

Aim of the Experiment

To determine the refractive index of the material of a given prism using a spectrometer.

Principle

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 the 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.It is given by snell’s law

Materials required

A spectrometer, a spirit level, a source of monochromatic light (sodium

vapour lamp), a glass prism, a magnifying lens and a reading lamp.

Procedure

(A) Telescope adjustment:

1. The spectrometer and the prism table are arranged in horizontal position by using

the levelling screws (as shown in fig. 2).

2. The telescope is turned towards a distant object to receive a clear and sharp image.

3. The slit is illuminated by a sodium vapour lamp and the slit and the collimator are

suitably adjusted to receive a narrow, vertical image of the slit.

4. The telescope is turned to receive the direct ray, so that the vertical slit coincides

with the vertical crosswire

(B) Measurement of the angle of the prism:

1. 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.

(C) Measurement of the angle of minimum deviation:

1. Place the prism so that its centre coincides with the centre of the prism table and

light falls on one of the polished faces and emerges out of the other polished face,

after refraction. The telescope is turned to view the refracted image of the slit on

the other face (as shown in fig. 4).

2. The vernier table is slowly turned in such a direction that the image of slit is

move directed towards the directed ray; ie., in the direction of decreasing angle of

deviation.

3. It will be found that at a certain position, the image is stationary for some moment.

The Vernier table is fixed at the position where the image remains stationary. Using

telescope fifine adjusting slider, make coincide the slit with cross wire.

4. Note corresponding main scale and vernier scale reading in both vernier (vernier I

and vernier II).

5. Carefully remove the prism from the prism table. Turn the telescope parallel to

collimator, and note the direct ray readings.

6. Find the difference between the direct ray readings and deviated readings. This

angle is called angle of minimum deviation (δm). Refractive index of the material

of the prism is determined by using eq.(12)

Observations:

1. Least count of spectrometer

One main scale division, N =. . . . . .degrees

No. of divisions on vernier, v =. . . . . .

Least count= N/v =. . . . . .degrees =. . . . . .seconds

2. Wavelength of light used = . . . . . . . . . .

3. Table for the angle of the prism (A):

4. Table for the angle of minimum deviation:

Results:

1. Angle of prism : . . . . . . . . . .degrees

2. Angle of minimum deviation of the prism : . . . . . . . . . .

3. Refractive index of the material of the prism : . . . . . . . . . .

4. Percentage Error :

Reference Material

1. Amritha University Virtual Lab: https://vlab.amrita.edu/?sub=1&brch=195&sim=840&cnt=1

2. PHET Virtual Lab: https://phet.colorado.edu/en/simulation/legacy/photoelectric

Questions

1. What is monochromatic light?

2. What is angle of prism?

3. What is angle of minimum deviation?

4. What is refractive index? What are its units?

5. What is the relation between the angle of incidence and the angle of deviation?