A piece of iron is heated in a flame. It first becomes dull red then becomes reddish yellow and finally turns to white hot. The correct explanation for the above observation is possible by using.

Options

A. Stefan's law
B. Wien's displacement law
C. Kirchhoff's law
D. Newton's law of cooling

Answer

To solve the question regarding the color change of a piece of iron as it is heated, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Phenomenon**: When a piece of iron is heated, it emits light at different wavelengths depending on its temperature. As the temperature increases, the color of the iron changes from dull red to reddish yellow and finally to white hot.

2. **Identifying the Laws of Physics**: The observation can be explained using a principle that relates temperature to the color of the emitted light. The relevant law here is Wien's Displacement Law.

3. **Wien's Displacement Law**: This law states that the wavelength of the radiation of maximum intensity emitted by a black body is inversely proportional to its temperature. Mathematically, it is expressed as:
\[ \lambda_m T = b \]
where \( \lambda_m \) is the wavelength of maximum intensity, \( T \) is the absolute temperature, and \( b \) is a constant.

4. **Application of the Law**: As the temperature of the iron increases:
- At lower temperatures, the iron emits longer wavelengths, which correspond to red light (dull red).
- As the temperature increases, the peak wavelength shifts to shorter wavelengths, resulting in the emission of yellow light (reddish yellow).
- Finally, at very high temperatures, the iron emits even shorter wavelengths, producing white light (white hot).

5. **Conclusion**: The change in color of the heated iron is a direct result of the increase in temperature and the corresponding shift in the wavelength of maximum intensity of the emitted radiation, as described by Wien's Displacement Law.

### Final Answer:
The correct explanation for the observation is provided by **Wien's Displacement Law** (Option B).

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