Question

When a convex lens is placed in water, its focal length:

• A. Increases
• B. Decreases
• C. Remains constant
• D. Becomes infinite

Hint:

When lenses are submerged in water, their focal lengths increase due to the change in refractive index.

Answer 1

The Correct Option is A

Step 1: Formula for focal length of a lens.
The focal length \( f \) of a lens in a medium is given by the formula: \[ \frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \] where \( \mu \) is the refractive index of the lens material relative to the medium. Step 2: Effect of water.
When a convex lens is placed in water, the refractive index of water affects the focal length. The refractive index of water is less than that of air, which increases the focal length of the lens. Step 3: Conclusion.
Hence, the focal length increases when the lens is placed in water.