An equiconvex lens of refractive index 1.5 and focal length 10 cm is held with its axis vertical and its lower surface immersed in water, the upper surface being in air. At what distance from the lens will a vertical beam of parallel rays incident on the lens be focussed? `mu. = 4//3`.

Explanation

To solve the problem, we need to determine the distance from the lens where a vertical beam of parallel rays will be focused when the lower surface of the lens is immersed in water.

### Step-by-Step Solution:

1. **Identify Given Data:**
- Refractive index of the lens, \( \mu_2 = 1.5 \)
- Focal length of the lens in air, \( f = 10 \, \text{cm} \)
- Refractive index of air, \( \mu_1 = 1 \)
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