Question

If a convex lens is placed in water, it's focal length

• A. increases
• B. decreases
• C. does not change
• D. none of the above

Answer 1

The Correct Option is A

To solve the problem, we need to understand how the focal length of a convex lens changes when placed in water.

1. Focal Length and Refractive Index:
The focal length of a lens is related to the refractive index of the lens material and the medium in which the lens is placed. The formula for the focal length of a lens in a medium is:

$ \frac{1}{f} = (n_{\text{lens}} - n_{\text{medium}}) \times \frac{1}{R} $

Where $n_{\text{lens}}$ is the refractive index of the lens material, $n_{\text{medium}}$ is the refractive index of the medium (water in this case), and $R$ is the radius of curvature of the lens.

2. Effect of Placing the Lens in Water:
When a convex lens is placed in water, the refractive index of the medium changes. Water has a refractive index of approximately 1.33, which is lower than the refractive index of glass (the common material of a convex lens).
Thus, the difference between the refractive index of the lens and the water decreases, resulting in an increase in the focal length.

Final Answer:
If a convex lens is placed in water, its focal length $ \mathbf{increases} $.