PHYSICS EXPERIMENT 3

EXPERIMENT NO. 3

Object: - To determine the dispersive power of the material of the prism using mercury light with the help of a spectrometer.

Apparatus required: - Spectrometer, prism, mercury Source and reading lens.

Theory:

(a)Diagram:-

(b)                Formula used: - The dispersive Power ‘w’ of the material of the prism is given by the formula

                                     

  Where,  m_v = refractive index of the material of the prism for violet colour,

                         m_r = refractive index of the material of the prism for red colour,

                                 The refractive index of the prism is given by

                                       

Where, A = Angle of the prism,      dm = Angle of minimum deviation

Procedure:

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 at the center. In this case some of the light failing on each face will be reflected and can be received with the help of the telescope.

3.        The telescope is moved to one side to receive the light reflected from the face AB and the cross wire 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 from the face AC and again the cross wire are focused on the image of the slit. The readings of two vernier are noted.

5.        The angle through which the telescope is moved or the difference in the two positions gives twice the refracting angle A of the prism. Therefore, half of this angle gives the refracting angle of the prism.

MEASUREMENT OF THE ANGLE OF MINIMUM DEVIATION

1.        Place the prism so that its centre coincide 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. In this position the spectrum of light is obtained.

2.        The spectrum is seen through the telescope is adjusted for minimum deviation position for a particular colour (wavelength) in the following way: -

              Setup telescope at a particular colour and rotate the prism table in one direction, of course the telescope should be moved in such way to keep the spectral line in view. By doing so a position will come where the spectral line recedes in the opposite direction although the rotation of the table is continued in the same direction. The particular position where the spectral line begins to recede in opposite direction is the minimum deviation position for that colour. Note the reading of the two verniers.

3.        Remove the prism table and bring the telescope in the line of the collimeter. See the slit directly through telescope and coincide the image of slit with vertical crosswire. Note the readings of two verniers.

4.        The difference in minimum deviation position and direct position gives the angle of minimum deviation for that colour.

5.        The same procedure is repeated to obtain the angles of minimum deviation for other colors.

 

Ovservation table

                             Table for angle [A] of the prism

S.No.	Vernier	Telescope reading for reflection from first face			Telescope reading for reflection from second face			2A= a~b                              deg.	Mean  2A deg.	A deg.
		M.S. deg.	V.S. deg.	TOTAL  (a) deg.	M.S. deg.	V.S. deg.	TOTAL (b) deg.
	V1
	V2

Table for angle of minimum deviation d_m

S.No	Color	Vernier	(Dispersed image) Telescope in minimum deviation position			Telescope reading for direct image			Deviation a~b	Mean deviation dm deg
			M.S. deg.	V.S. deg.	TOTAL (a) deg.	M.S. deg.	V.S. deg.	TOTAL (b) deg.
1	Violet	V1
		V2
2	Yellow	V1
		V2
3	Red	V1
		V2

Calculation:-                         

                                     

Result:

Dispersive power of material of the prism ω=............

Percentage error:

                                                                                                                                                                  (Standard value~ Observed Value) X 100

                  % ERROR =

                                                                    Standard Value

Precautions :-

1.            The telescope and collimator should be individually set for parallel rays.

2.            Slit should be as narrow as possible.

3.            While taking observations, the telescope and   prism table should be clamped with the help of clamping screws.

4.            Both verniers should be read.

5.            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.

Viva Voce

(1)   What is prism?

Ans : A transparent medium like glass bounded by two triangle and   three rectangular smooth surfaces

 (2) Which colour in the spectrum is having maximum and minimum refractive index?

      Ans : Maximum for Violet Colour

                Minimum for red colour.

   (3) What is Refractive index?         

 Ans : The ratio of sine of angle of incidence in the first medium to the sine of angle of refraction in the second medium.

(4)What is the function of Collimator?

 Ans : It will produce parallel beam of light.

 (5)What do you mean by Angle of Prism?

Ans : Angle between two refracting surfaces of the prism.

 (6)What is Dispersion of Light?

 Ans : When the light is allowed to fall on one of the refracting surfaces of a prism , it is split into its constituent colours. This splitting of light into its constituent  colours by refraction through prism is called Dispersion of light.

(7)What is the main optical action of the prism?

 Ans : The main optical action of a prism is to disperse white light into its component parts. Dispersion of light is minor optical action of prism , but main effect of a prism is to deviate a beam of light.

 (8)What type of material prism is used in this experiment?

 Ans : Crown prism.

 (9)What is the units of Dispersive power?

 Ans : No units.

 (10)What type of light do you use in this experiment?

 Ans : White light with the help of mercury lamp.