(a) When a convex lens of focal length 30 cm is in contact with a concave lens of focal length 20 cm, find out if the system is converging or diverging.
(b) Obtain the expression for the angle of incidence of a ray of light which is incident on the face of a prism of refracting angle A so that it suffers total internal reflection at the other face. (Given the refractive index of the glass of the prism is .)
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Given,
The focal length of the convex lens
The focal length of the concave lens
Now, the combined focal length
will be ;
Hence, the combination works as a diverging lens since F is negative.
(b) To determine the angle of the incident ;
We know,
Such that,
Since,
(By refraction)