Question

If the space between the lenses in the lens combination shown were filled with water, what would happen to the focal length and power of the lens combination?

	Focal length	Power
(a)	Decreased	increased
(b)	Decreased	unchanged
(c)	Increased	unchanged
(d)	Increased	decreased

• A. a
• B. b
• C. c
• D. d

Answer 1

Answer: (D)

The focal length of the plano convex lens is +ve. Its power is +ve too, say $+P_1$. When the gap is filled with water, the shape of the lens is biconcave. It acts as a concave lens whose focal length is -ve and power is -ve too, say $\left(-P_2\right)$ Thus $P_{\text{combination }}=P_1+\left(-P_2\right)+P_1=2P_1-P_2$
So when filled with water, the power decreases, hence focal length increases.