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NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions

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Question : 4
Total: 38
Figures (a) and (b) show refraction of a ray in air incident at 60° with the normal to a glass-air and water-air interface, respectively. Predict the angle of refraction in glass when the angle of incidence in water is 45° with the normal to a water-glass interface (figure c).

Solution:  
(a) Applying Snell’s law for the refraction from air to glass.
Refractive index of glass w.r.t. air
a
‌
µg =
sin60°
sin35°
 =
0.8660
0.5736
 = 1.51
(b) Now Snell’s law for the refraction from air to water
a
‌
µω =
sin60°
sin47°
 =
0.8660
0.6560
 = 1.32
(c) Now the light beam is incident at an angle 45° from water to glass
ω
‌
µg =
sin45°
sinr
 ⇒
1.51
1.32
 =
sin45°
sinr
 =
0.7071
sinr
sin r =
13.2×0.7071
1.51
 = 0.6181
∴ r = 38.2°
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