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Laser Diffraction

The document outlines an experimental procedure to determine the wavelength of unknown light lines using laser diffraction. It details the working principle involving diffraction grating, the apparatus required, and the steps to measure diffraction angles. Additionally, it includes tables for recording experimental results and calculating the mean wavelength and percentage error.

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55 views4 pages

Laser Diffraction

The document outlines an experimental procedure to determine the wavelength of unknown light lines using laser diffraction. It details the working principle involving diffraction grating, the apparatus required, and the steps to measure diffraction angles. Additionally, it includes tables for recording experimental results and calculating the mean wavelength and percentage error.

Uploaded by

s36347032
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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LASER DIFFRACTION

Aim: To determine the wavelength of given unknown lines by laser diffraction method
Working Principle
If a parallel beam of light of wavelength is coming out from diffraction grating, placed
vertically on the optical bench, then the diffracted rays from the grating will form at the focal
plane, a number of primary maxima of different order numbers (n) on both sides of the central
maximum of zero order. If be the angle of diffraction of nth order primary maximum then

where is the number of rulings per cm of the grating surface. Hence, the
wavelength of the incident light is,

where the angle of diffraction of nth order maxima


no. of rulings per cm of the grating surface
order number of primary maxima
Apparatus Used:
Laser (He-Ne) source
Diffraction grating
Laser spectrometer

Experimental Results
Table 1: To measure diffraction angles for different orders
A. Readings for the vernier constants of both verniers of the spectrometer

Value of 1 smallest main scale division (m.s.d)= ------------ .

(n) vernier scale division = (m) main scale divisions


B.
V Readings for the diffracted images with the telescope at
e Left Right Difference
r M.S.R V.S.R Total M.S.R V.S.R Total between
n in deg,min in deg, in in deg, the left
Order i min, deg,min min, and the
Mean
no. N e sec sec right
r
readings,
n
2
o
.
1.
1
2
1.
2
2
1.
3
2

Value of M = ---------------------(lines/ mm)

Table2: Estimation of wavelength

Value of Mean wavelength


Order no. ( N )
cm (nm)
(from table 1B)

1.
2.

3.

Percentage of Error

P.E=

=angle of diffraction from vernier1

angle of diffraction from vernier2

Procedure:

1.​ Level the spectrometer with spirit level.

2.​ Determine the vernier constant of the spectrometer.

3.​ Switch on the laser source. See the spot on the wall. Bring the detector on the optical
path. You may have to look from the source side to see the spot on the detector. Adjust
the horizontal position of the detector to obtain a maximum reading in the associated
panel. .Note down the position of the direct beam. This helps you to connect the order of
the diffraction spots.

4.​ Set the grating on the prism table normal to the laser beam with careful eye estimation.

5.​ Remove the detector from the optical path by rotating its arm and observe the spectrum
on the wall of the room facing the source.
6.​ Now bring back the detector on the path and place the detector at different positions of
the spots. Measure the angle from both the vernier scales.

DISCUSSION:

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