Discussion
A convex lens in air has focal length f. If it is immersed in water, its focal length will be
Answer: A
Its focal length is Increased.
By the lens-maker's formula, the power of a lens depends on how much its refractive index exceeds that of the medium around it:
\[ \begin{aligned} \frac{1}{f} &= \left(\frac{n_{g}}{n_{m}} - 1\right) \\ &\quad \times \left(\frac{1}{R_{1}} - \frac{1}{R_{2}}\right) \end{aligned} \]
For glass (1.5) in air the bracket is 0.5; in water (1.33) it is only about 0.125. The lens bends light less, so its focal length becomes about four times longer. It does not decrease, stay the same or become zero.
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