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Question : 26
Total: 26
(a) Define wavefront. Use Huygens' principle to verify the laws of refraction.
(b) How is linearly polarised light obtained by the process of scattering of light? Find the Brewster angle for air - glass interface, when the refractive index of glass= 1 ⋅ 5.
OR
(a) Draw a ray diagram to show the image formation by a combination of two thin convex lenses in contact. Obtain the expression for the power of this combination in terms of the focal lengths of the lenses.
(b) A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is
(b) How is linearly polarised light obtained by the process of scattering of light? Find the Brewster angle for air - glass interface, when the refractive index of glass
OR
(a) Draw a ray diagram to show the image formation by a combination of two thin convex lenses in contact. Obtain the expression for the power of this combination in terms of the focal lengths of the lenses.
(b) A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is
Solution:
(a) Definition of wavefront Verifying laws of refraction by Huygen's principle
(b) Polarisation by scattering Calculation of Brewster's angle
(a) The wavefront is the common locus of all points which are in phase(surface of constant phase)
Let a plane wavefront be incident on a surface separating two media as shown. Letv 1 v 2 B C = v 1 t and A D = v 2 t
s i n i =
and s i n r =
∴
=
=
=
a constant
This proves Snell's law of refraction.
(b) When unpolarised light gets scattered by molecules, the scattered light has only one of its two components in it. (Also accept diagrammatic representation)
We have, µ = tan i B
∴ tan i B = 1.5
∴ i B = tan − 1 1.5 = 56.3 ∘
OR
(a) Ray diagram
Expression for power
(b) Formula
Calculation of speed of light
Two thin lenses, of focal lengthf 1 f 2 O u I 1 v 1
This image serves as object for the second lens. Let the final image be at I. We then have
=
−
=
−
Adding, we get
+
=
−
=
∴
=
+
P = P 1 + P 2
(b) At minimum deviation
r = A ∕ 2 = 30 ∘
We are given that
i =
A = 45 ∘
∴ µ =
= √ 2
∴ =
( ≅ 2.1 × 10 8 ms − 1 )
(b) Polarisation by scattering Calculation of Brewster's angle
(a) The wavefront is the common locus of all points which are in phase(surface of constant phase)
Let a plane wavefront be incident on a surface separating two media as shown. Let
This proves Snell's law of refraction.
(b) When unpolarised light gets scattered by molecules, the scattered light has only one of its two components in it. (Also accept diagrammatic representation)
OR
(a) Ray diagram
Expression for power
(b) Formula
Calculation of speed of light
Two thin lenses, of focal length
This image serves as object for the second lens. Let the final image be at I. We then have
Adding, we get
(b) At minimum deviation
We are given that
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