On a hot sunny day in deserts, illusion of water is seen at some distance. Explain.

Explanation

### Step-by-Step Solution:

1. **Understanding the Phenomenon**: The illusion of water seen on hot sunny days in deserts is known as a "mirage". This optical phenomenon occurs due to the refraction of light rays in layers of air at different temperatures.

2. **Temperature Gradient**: On a hot day, the surface of the sand becomes very hot. This heat warms the air directly above the sand, creating a temperature gradient where the air closest to the ground is much hotter than the air above it.

3. **Refractive Index**: The refractive index of air decreases with an increase in temperature. Therefore, the hot air near the ground has a lower refractive index (is a rarer medium) compared to the cooler air above it (which is a denser medium).

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