Show that for refraction at a concave spherical surface (separating glass-air medium), the distance of the object should be greater than three times the radius of curvature of the refracting surface for the image to be real.

Explanation

To show that for refraction at a concave spherical surface (separating glass and air), the distance of the object should be greater than three times the radius of curvature of the refracting surface for the image to be real, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Refractive Indices:**
- Let the refractive index of glass (medium 1) be \( \mu_1 = 1.5 \).
- Let the refractive index of air (medium 2) be \( \mu_2 = 1.0 \).

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