# (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 .)

Given,

The focal length of the convex lens

The focal length of the concave lens

Now, the combined focal length  will be ;

( concave lens focal length =-20cm)

Hence, the combination works as a diverging lens since F is negative.

(b) To determine the angle of the incident ;

We know,

(Where,  refractive prism)

Such that,

____(1)

Since,              (By refraction)

(From eqn (1))

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