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```					St. Mark’s School
Name: ______________________ Group: _______
TAS Experiment:
Class: ______________________ No.:__________
Estimation of the wavelength of green and red
Date: ______________________________________
light by using a plane diffraction grating
Mark: _____________________________________
Aim:        To measure the wavelength of green and red light by a plane diffraction grating.

Apparatus:             Ray box with vertical filament                     1
12 V low voltage power supply                      1
Diffraction grating (80 lines per mm)              1
Green colour filter                                1
Metre rule                                         1
Half-metre rule                                    1
Long pin with holder                               1
Grating holder                                     1
Opaque adhesive tape 4 cm                          2
Red LED                                            1
1100  resistor                                    1
Connecting wires

Metre rule B

Ray box with vertical
filament lamp few metres
away from the grating
Metre rule A

Figure 1

Procedure:
1. Set up the apparatus as shown in Figure 1.
2. Put metre rule A along the direction of white light. Metre rules B must be perpendicular to metre rule
A.
3. Mount the grating (80 lines per mm) on the grating holder. DO NOT touch the rulings of the grating
plate.
Slit separation d = ___________________

4.   Switch on the lamp. Check if the filament of the lamp is placed in the vertical direction and the
grating is placed directly facing the ray box with its rulings vertical.

5.   View through the grating. Draw the image observed. (2 marks)

Page 1
6.  Put a green filter in front of the light bulb.
7.  Ask your partner to move the long pin along metre rule B until its position overlaps with the middle
of the green band of the first order spectrum.
8. Measure the distance between the metre rule A and the long pin.
9. Repeat steps 7 and 8 for the second order of green light.
10. Complete the following table.
11. Calculate the angle of diffraction for each maxima.

Order of maxima                      First Order                Second Order
Distance between metre
rule A and the long pin.

Angle of diffraction θ
(Show working steps)

from ‘B’
grating

θ

light
~1 m

12. Write down the expression of λ (wavelength) in terms of m (order of maxima), θ (diffraction
angle) and a (slit width).

13 . For m = 1 and 2, calculate the wavelength of the green light respectively.

1

2

Mean value ofλ=                                                                   (2 marks)

14. Connect the red LED according to the following circuit diagram and repeat the experiment by
replacing the ray box with the LED.

.

1100

3V

Note: Make sure the LED is connected in forward bias.

Page 2
Order of maxima                First Order    Second Order

Distance between metre rule A
and the long pin.
Angle of diffraction θ

Wavelength of red light

(1 mark)
Questions:

1. Explain how did you place the ruler A in the direction of white light from the ray box and
perpendicular to ruler B. (1marks)

2. How the results will be affected if the diffraction gating is replaced by a 300 lines per mm one?
Which grating will give a higher accuracy of the wavelength? Explain your answer. (2 marks)

3. Account for the sources of error in the experiment.(2 marks)

Page 3

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