A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear?

Explanation

To solve the problem of determining the refractive index of the liquid required for a glass lens (with a refractive index of 1.5) to become invisible, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept**:
- A lens appears to disappear when the light rays passing through it do not bend at the interface between the lens and the surrounding medium. This occurs when the refractive index of the lens is equal to that of the surrounding medium.

2. **Identify the Given Information**:
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