When a ray of light enters from one medium to another having different optical densities, it bends. Why does this phenomenon occur ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Light Propagation**: Light travels in straight lines in a uniform medium. However, when it encounters a boundary between two different media (like air and water), its behavior changes.

2. **Optical Density**: Optical density refers to how much a medium can slow down light. Different materials have different optical densities. For example, air has a lower optical density compared to water or glass.

3. **Change in Speed**: When light moves from one medium to another with a different optical density, its speed changes. Light travels faster in less dense media and slower in denser media.

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