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 1: Understand the Setup
We have a concave spherical surface separating two media: glass (with refractive index \( \mu_1 \)) and air (with refractive index \( \mu_2 \)). The object is placed in the glass medium.

### Step 2: Define the Variables
- Let \( R \) be the radius of curvature of the concave surface.
- Let \( u \) be the object distance (measured from the surface).
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