| a. |
Give
a brief account on the differences between light emitted from an ordinary
light bulb and light from a Laser. |
3
marks |
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| Ordinary light |
Light from Laser |
| Consists of a range (spectrum) of color |
Monochromatic |
| Bursts of wave trains |
Continuous waveform |
| Short coherent length (sudden phase change within a wave
train of light) |
Coherent |
| Divergent |
Uni-directional |
| Weak |
Strong |
|
Any 3 |
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| b. |
Discuss
how the wavelength of light from a sodium lamp could be measured from the
interference pattern produced by a Young’s double-slit. Discuss the conditions
for producing such an interference pattern. |
5
marks |
| |
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 |
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Light passing through the narrow slits at a small separation
a behaves like two coherent sources. |
|
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The path difference for the two rays on reaching P
is |
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 |
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If the mth bright fringe occurs at P, then |
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 |
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The distance of P from the central maximum is |
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 |
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For small angle q, we have |
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 |
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Therefore, |
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 |
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By knowing the values of m, D, a and
ym, the wavelength l could
be worked out. |
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The conditions for obtaining an observable fringe pattern
with measurable fringe separation are |
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-
The slit separation is small, for instance, in the order of the wavelength
of light
-
The slits are narrow so that light diffracts around the slit
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| c. |
Explain
the term “optical path”. Hence, discuss the effect on the interference
pattern produced by a Young’s double-slit if the whole arrangement is immersed
in water. |
4
marks |
| |
|
|
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Optical path of light in a medium is the equivalent distance
travelled by light in vacuum. |
1 |
|
The wavelength of light in water is less than
that in vacuum. |
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 |
1 |
|
where n is the refractive index of water. |
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Thus, for a given physical length t
in water, the optical path is |
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optical path = n
t |
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When the set-up is immersed in water, the optical
path difference at the mth bright fringe is |
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 |
1 |
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Thus, the position becomes |
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 |
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It is at a lower position than P. i.e.
the fringes are closer together. |
1 |
| |
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| d. |
What
is the effect of adding more slits to a Young’s double-slit? Illustrate
your answer by referring to a 3-slit plate with uniform slit separation. |
4
marks |
| |
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 |
2 |
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When one more slit is added to a double-slit,
the positions of the maxima are unchanged. However, in between any two
maxima, a peak is formed. |
1 |
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In Fig.b, light from A and B
are p out of phase and cancel out (destructive
interference). This leaves light from C alone. The intensity of
the peak is only 1/9th of that at the maxima. |
1 |
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