What is meant by the term 'critical angle'?

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Concept**: The term 'critical angle' refers to a specific angle of incidence in optics. It occurs when light travels from a denser medium to a rarer medium.

2. **Definition**: The critical angle is defined as the angle of incidence in the denser medium at which the angle of refraction in the rarer medium is 90 degrees.

3. **Visualizing the Scenario**: Imagine two different media: one is denser (for example, glass) and the other is rarer (for example, air). When light travels from the denser medium to the rarer medium, it bends away from the normal line (the line perpendicular to the surface at the point of incidence).

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