Test Index

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

© examsnet.com
Question : 6
Total: 38
A prism is made of glass of unknown refractive index. A parallel beam of light is incident on a face of the prism. The angle of minimum deviation is measured to be 40°. What is the refractive index of the material of the prism? The refracting angle of the prism is 60°. If the prism is placed in water (refractive index 1.33), predict the new angle of minimum deviation of a parallel beam of light.
Solution:  
When the light beam is incident from air on to the glass prism, the angle of minimum deviation is 40°.
Refractive index of glass w.r.t. air.

a
‌
µg =
sin
A+δm
2
sin
A
2
a
‌
µg =
sin
60°+40°
2
sin
60
2
 =
sin50°
sin30°
 =
0.766
0.50
 = 1.532
Now the prism is placed in water, new angle of minimum deviation can be calculated.

ω
‌
µg =
sin
A+δm′
2
sin
A
2
a
‌
µg
a
‌
µω
 =
sin
60+δm′
2
sin30°
sin (30°+
δm′
2
) =
1
2
[
0.532
1.33
] = 0.5759
30° +
δm′
2
 = sin−1 (0.5759) or 30° +
δm′
2
 = 35° 10'
New angle of deviation δ′m = 10° 20′
© examsnet.com
Go to Question:
Prev Question
Next Question