Question

When a convex lens is dipped in water, its focal length and nature become:

• A. increased, convex lens
• B. decreased, convex lens
• C. increased, concave lens
• D. decreased, concave lens

Hint:

Use the lens maker's formula: in water the factor \( (\mu_g/\mu_w - 1) \) is smaller but still positive, so \( f \) grows while the lens stays converging.

Answer 1

The Correct Option is A

Step 1: Focus on the refractive-index difference.
A lens bends light because of the difference between the lens index and the medium index. The larger this difference, the stronger the lens and the shorter its focal length.

Step 2: Move from air to water.
Water (\(\mu \approx 1.33\)) is optically closer to glass (\(\mu \approx 1.5\)) than air (\(\mu = 1\)) is. So immersing the lens in water shrinks the index difference from about \(0.5\) to about \(0.17\).

Step 3: Effect on strength and focal length.
A smaller index difference means a weaker lens. A weaker converging lens has a longer focal length, so \(f\) increases.

Step 4: Effect on nature.
Because glass is still denser than water, the lens continues to converge light. It does not turn into a diverging (concave-acting) lens; that reversal needs a medium denser than the lens.

Step 5: Answer.
Thus in water the convex lens keeps its converging (convex) character but with a larger focal length.
\[\boxed{f \text{ increases, lens stays convex}}\]