The ratio of the Emissive power to the absorption power of all substances for a particular wavelength is the same at given temperature. The ratio is known as

Options

A. the emissive power of a perfectly black body
B. the emissive power of any type of body
C. the Stefan's constant
D. the Wien's law

Explanation

To solve the question regarding the ratio of the emissive power to the absorption power of all substances for a particular wavelength at a given temperature, we can follow these steps:

### Step 1: Understand the Terms
- **Emissive Power (E)**: This is the ability of a substance to emit thermal radiation.
- **Absorptive Power (A)**: This is the ability of a substance to absorb thermal radiation.
- The ratio of emissive power to absorptive power is given by \( \frac{E}{A} \).

### Step 2: Recall Kirchhoff's Law of Heat Radiation
According to Kirchhoff's law, at thermal equilibrium, the ratio of emissive power to absorptive power for any substance at a given temperature is constant and is equal to the emissive power of a black body at that temperature.

### Step 3: Define the Black Body
A black body is an idealized physical object that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Its absorptive power is 1, meaning it absorbs all radiation.

### Step 4: State the Ratio
From Kirchhoff's law, we can express the relationship as:
\[ \frac{E}{A} = \text{Emissive Power of Black Body} \]
Since the absorptive power of a black body is 1, the emissive power of a black body is equal to its absorptive power.

### Step 5: Conclude the Answer
Thus, the ratio of the emissive power to the absorption power of all substances for a particular wavelength at a given temperature is known as the **Emissive Power of a Black Body**.

### Final Answer
The correct answer is **Emissive Power of Black Body**.

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