State kirchhoff's law of blakc radiations.

Answer

**Step-by-Step Solution:**

1. **Understanding Kirchhoff's Law of Radiation**: Kirchhoff's law relates to the behavior of thermal radiation emitted and absorbed by bodies at thermal equilibrium.

2. **Constant Temperature Requirement**: The law is applicable at a constant temperature. This means that the temperature of the body must not change during the process of emission and absorption of radiation.

3. **Coefficient of Absorption**: The coefficient of absorption (denoted as \( \alpha \)) is a measure of how much radiation a body can absorb. It ranges from 0 to 1, where 0 means no absorption and 1 means complete absorption.

4. **Coefficient of Emission**: The coefficient of emission (denoted as \( \epsilon \)) is a measure of how much radiation a body emits. Similar to absorption, it also ranges from 0 to 1.

5. **Statement of Kirchhoff's Law**: Kirchhoff's law states that at a constant temperature, the coefficient of absorption of a body is equal to its coefficient of emission. Mathematically, this can be expressed as:
\[ \alpha = \epsilon \]

6. **Conclusion**: Therefore, Kirchhoff's law of black radiation emphasizes that for a body in thermal equilibrium at a constant temperature, the ability to absorb radiation is equal to the ability to emit radiation.

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