NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions
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Question : 22
Total: 38
At what angle should a ray of light be incident on the face of a prism of refracting angle 60° so that it just suffers total internal reflection at the other face? The refractive index of the material of the prism is 1.524
Solution:
The beam should be incident at critical angle or more than critical angle, for total internal reflection at second surface of the prism.
Let us first find critical angle for air glass interface.
We know
sin C =
C =s i n − 1 (
) s i n − 1 (
)
Now we can calculate ‘r’, as
60° + (90° – r) + (90° – C) = 180° or, r = 19°
Using Snell’s law, required angle of incidence i at first surface can be calculated.
µ g
sin i = 1.524 (sin 19°)
or i =s i n – 1
Let us first find critical angle for air glass interface.
We know
sin C =
C =
Critical angle C = 41°
Now we can calculate ‘r’, as
60° + (90° – r) + (90° – C) = 180° or, r = 19°
Using Snell’s law, required angle of incidence i at first surface can be calculated.
sin i = 1.524 (sin 19°)
or i =
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