prepared by khursheed ahmed cheshtti from central university amarkantak mp

1. Lab Seminar
on
dispersive power of prism
By
1. Khursheed ahmed M.Sc. (physics) sem-II
2. Joyti ranjan M.Sc. (Physics) sem –II

2. • INTRODUCTION
• APPARATUS REQUIRED
• IMPORTANT FORMULAES
• THEORY
• PROCEDURE
• PRECAUTIONS
• RESULTS
• CONCLUSION
Outlines:

3. INTRODUCTION:-
 Spectrometer. It is a compact apparatus for obtaining a
pure spectrum. It is used for the study of spectra and for
finding the refractive index of the material in the form of
prism.
 It has three parts
 1.The collimator.(C). Its purpose is to produce a parallel
beam of light.
 2.The telescope (T). It is an astronomical telescope having
achromatic objective and a Ramsden’s eye piece.
 3. The prism table. It is a table between the collimator
and telescope which is used to place prism.

4. .APPARATUS REQUIRED:-
 A SPECTROMETER, A SPIRIT LEVEL, A SOURCE OF MONOCHROMATIC LIGHT (SODIUM LAMP) , A GLASS
PRISM, A WOODEN SCREEN, AN EYE PIECE, AND AN ELECTRIC LAMP.
IMORTANT FORMULAS:-The dispersive power of prism is given by
 Where µ2 = Refractive index for the violet rays
µ1 = Refractive index for red light
µ = mean

5. Refractive index is given by:-
Where A = Angle of prism
δm = Angle of minimum deviation

6. Dispersion of light:-
when a ray of white light passes through a prism it splits up into
its constituents colors this phenomenon of splitting of white light
its constituent color is called dispersion .
Angle of prism: The angle between two surfaces of the prism
is known as the refracting angle or angle of prism.
ANGLE OF MINIMUM DEVIATION: A ray of light is deflected twice in a
prism. The sum of these deflection is the deviation angle.

7. PROCEDURE:-
1. Find the angle of prism by rotating the telescope method.
2. Place the prism on the prism table so that its center coincide with the table.
3. Look through The prism and locate the position of refracted image with naked eye, bring the
telescope in this position adjust its position till the image falls on its crosswire rotate the
telescope and bring on the line of collimator axis.
Telescope should be perpendicular the collimator after viewing the image note down the reading
on both the scale.
4. keep the table fix and remove the prism gently rotates
the telescope and bring it with the line in collimator and note
the reading on the scales v1 and v2.
5. Repeat the experiment and take such three readings.

8. Observation tables:- To find the angle of prism
Mean A=
To find the refractive index the observation table will be as follows:
Vernier v1 Vernier v2
S.No Telescope reading Difference θ A = θ/2 Telescope reading Difference θ A = θ/2
1st face 2nd face 1st face 2nd face
1
2
3
S.No Vernier V1 Vernier V2
Telescope reading Telescope reading
Min. dev. Direct Difference Dm Min. dev. Direct Difference Dm
1
2
3

9. Calculation:
Assumed that A= 60°
And minimum deviation dm= 35°
Then putting these values in the above formula we get refractive index = 1.46
Once the refractive index is known for different color of light we can easily find the values of
Dispersive power of prism by putting the values in the given formula.

10. Precautions:-
 Do not touch the reflecting surface of the prism.
 Place the prism on the prism table gently and also remove with
proper care.
 The telescope should be focused with proper care.
 Collimator should be adjusted to give a parallel beam of light.
 The slit should be narrow.
 Do not move the telescope forcefully.
 Read both the verniers scale to avoid error.

11. Calculations and results
By finding the angle of prism A.
Angle of minimum deviation Dm
We can find the refractive index of the prism µ.
Finely the dispersive power of prism
i.e Ꞷ can be find by putting these values in the
given formula
Hence dispersive power of prism = --------------

12. Thank you