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Question : 25
Total: 26
(a) Distinguish between unpolarised light and linearly polarized light. How does one get linearly polarised light with the help of a polaroid?
(b) A narrow beam of unpolarised light of intensityI 0 P 1 P 2 60 ∘ P 1 P 2
OR
(a) Explain two features to distinguish between the interference pattern in Young's double slit experiment with the diffraction pattern obtained due to a single slit.
(b) A monochromatic light of wavelength500 nm 0.2 mm
Estimate the number of fringes obtained in Young's double slit experiment with angular fringe width 0.5 , which can be accommodated within the region of total angular spread of the central maximum due to single slit.
(b) A narrow beam of unpolarised light of intensity
OR
(a) Explain two features to distinguish between the interference pattern in Young's double slit experiment with the diffraction pattern obtained due to a single slit.
(b) A monochromatic light of wavelength
Estimate the number of fringes obtained in Young's double slit experiment with angular fringe width 0.5 , which can be accommodated within the region of total angular spread of the central maximum due to single slit.
Solution:
(a) Distinction between unpolarised and linearly polarized light 2 Obtaining linearly polarized
(b) Calculation of intensity of light
(a) In an unpolarised light, the oscillations of the electric field, are in random directions in planes perpendicular to the direction of propagation. For a polarized light, the oscillations are aligned along one particular direction.
Alternatively
Polarized light can be distinguished from unpolarised light when it is allowed to pass through a polaroid. Polarized light can show change in its intensity, on passing through a Polaroid; intensity remains same in case of unpolarised light.
When unpolarised light wave is incident on a polaroid, then the electric vectors along the direction of its aligned molecules, get absorbed; the electric vector, oscillating along a direction perpendicular to the aligned molecules, pass through. Thislight is called linearlv polarized light.
(b) According to Malus' Law:
I = I 0 cos 2 θ
I = (
) cos 2 θ ,
whereI 0
θ = 60 ∘ (given)
I =
cos 2 60 ∘ =
× (
) 2
=
OR
(a) Explanation of two features (distinguishing between interference pattern and diffraction pattern.)
(b) Calculation of angular width of central maxima
Estimation of number of fringes
(a)
(b) Angular width of central maximum
ω =
=
radian
= 5 × 10 − 3 radian
β =
Linear width of central maxima in the diffraction pattern
ω ′ =
Let 'n
∴ n × β = ω ′
n =
×
n =
[Note : Award the last1 ∕ 2 d = a
(b) Calculation of intensity of light
(a) In an unpolarised light, the oscillations of the electric field, are in random directions in planes perpendicular to the direction of propagation. For a polarized light, the oscillations are aligned along one particular direction.
Alternatively
Polarized light can be distinguished from unpolarised light when it is allowed to pass through a polaroid. Polarized light can show change in its intensity, on passing through a Polaroid; intensity remains same in case of unpolarised light.
When unpolarised light wave is incident on a polaroid, then the electric vectors along the direction of its aligned molecules, get absorbed; the electric vector, oscillating along a direction perpendicular to the aligned molecules, pass through. Thislight is called linearlv polarized light.
(b) According to Malus' Law:
where
OR
(a) Explanation of two features (distinguishing between interference pattern and diffraction pattern.)
(b) Calculation of angular width of central maxima
Estimation of number of fringes
(a)
| Interference Pattern | Diffraction pattern |
|---|---|
| (i) All fringes are of equal width. | (i) Width of central maxima is twice the width of higher order bands. |
| (ii) Intensity of all bright bands is equal. | (ii) Intensity goes on decreasing for higher order of diffraction bands. |
Linear width of central maxima in the diffraction pattern
Let '
[Note : Award the last
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