A body is kept inside a container the temperature of the body is `T_(1)` and the temperature of the container is `T_(2)` the rate at which body absorbs the energy is `alpha` The emissivity of the body is The radiation striking the body is either absorbed or reflected
A good absorber is .

Options

A. good reflector
B. poor reflector
C. average reflector
D. Assessment not possible

Answer

To solve the question about what defines a good absorber in the context of thermal radiation, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of Absorption and Emission**:
- A good absorber is a body that can absorb a significant amount of incident radiation. According to Kirchhoff's law of thermal radiation, a good absorber is also a good emitter.

2. **Defining Emissivity**:
- Emissivity (E) is a measure of how effectively a surface emits thermal radiation compared to a perfect black body. The value of emissivity ranges from 0 to 1, where 1 indicates a perfect emitter and absorber (black body).

3. **Applying Kirchhoff's Law**:
- Kirchhoff's law states that at thermal equilibrium, the emissivity of a body is equal to its absorptivity. Therefore, a good absorber has an emissivity close to 1, meaning it absorbs nearly all the incident radiation.

4. **Identifying Characteristics of a Good Absorber**:
- A good absorber will have a high absorption coefficient (close to 1) and a low reflectivity (close to 0). For example, a black body is considered a perfect absorber and emitter because it absorbs all incident radiation and reflects none.

5. **Conclusion**:
- Therefore, a good absorber is defined as a body that absorbs all incident radiation and reflects none, which corresponds to an emissivity of 1.

### Final Answer:
A good absorber is a body that has an emissivity of 1, meaning it absorbs all incident radiation and reflects none.

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