What is total internal reflection ? Under what conditions it takes place ?

Explanation

### Step-by-Step Solution:

1. **Definition of Total Internal Reflection**:
Total Internal Reflection occurs when a light ray traveling from a denser medium to a rarer medium strikes the boundary at an angle greater than the critical angle. In this case, instead of refracting into the rarer medium, the light ray reflects entirely back into the denser medium.

2. **Conditions for Total Internal Reflection**:
- **First Condition**: The light must be traveling from a denser medium to a rarer medium. This means the refractive index of the first medium (denser) must be greater than that of the second medium (rarer). Mathematically, this can be expressed as \( n_1 > n_2 \), where \( n_1 \) is the refractive index of the denser medium and \( n_2 \) is the refractive index of the rarer medium.

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