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Determination of ruling width ofa compact disc using laser
6. (lii) Determination of ruling width of a
compact disc using laser
bjective
To determine the ruling width ofa compact disc (CD) using laser source.
Materials and Apparatus
+ Laser Source
Meter
" Compact disc (CD)
Scale
"
Stand
heory
The ruling width (track spacings) of
given compact disc can be determined using the
rinciple of grating. The surface of compact disc has spiral grooved tracks and behind there is
highly reflective surface, which acts like a plane reflection grating. The spacing between the
lternate tracks is referred as ruling width (). The light reflected from the regions between the
dosely spaced tracks interfere constructively only in certain directions that depends on
wavelength and the direction of incident light. The diffraction pattern will be formed by the
eflectedlight from CD, as hown in the Fig, 6.5. The diffraction formula for normal incidence
nd = dstín.,
(1)
where n is the order of diffraction pattern, 8, is diffraction angle of the n order, d is the
ruling width on the CD and A is the wavelength of the laser beam. If L is the between the
CD and the screen and y is the distance of various order diffraction spots from the central
maxina, then
tan®, 9, =tan
(2)
By measuring y. and L, one can find the value of q Knowing the wavelength
of
Jaser source, the ruling width is calculated using the relation,
d=
sinb
(3)
Procedure:
1. Keep the He-Ne Laser behind the meter scale screen as shown in Fig. 6.5
2. Clamp the given CD vertically ina stand at a considerable height in front of the screen.
3. Turn on the laser. Carefully align the laser such that it passes through the hole made
in the metre scale.
4. Adjust the CD, such that the laser shines at outer region (Circumference) of the
surfaceof the CD where the tracks are approximately parallel.
5. To ensure that there is normal incidence of the laser light on the CD, carefully adiust
the CD until tbe central spot is reflected back onto the incident beam.
63
Department of Physics, Anna University, Chennai
6. Rotate the CD until the diffraction pattern (diffraction spots) is formed on the
scale and are lined horizontally.
7. Measure the distance L between the CD and screen.
8. From the centre spot (zeroh order), note the distances y. of different orders in the
scale on both sides.
9. Change the distance between the CD and screen and repeat the experiment.
Fig. 6.5
Precautions
1. Never view the laser light directly with naked eye.
Result
The ruling width of the compact disc is
Analytical Questions:
m.
1. What happens if the wavelength of the laser is changed?
2 If cD is replaced by a DVD, will there any change in the
diffraction pattern
3, How tracks are formed in a CD?
4 What happens to the
64
diffraction pattern if you increase the track width on a
R)
Determination
of ruling width of a
compact disc using laser
Table
Mteelength
of the light from He-Ne laser, A
Distance
Order
between the
Screen and
n
the CD (
cm
632.8 nm 632.8/% 10°m
Distance between zero
order And different
order spots (y)
Left
Right
Mean
cm
cm
cm
tan e,=
d=
L
sin®,
m
Mean =
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