AIIMS 2005 Physics Radiation Assertion Reason Question

To analyze the question thoroughly, let's break down both the assertion and the reason, and then evaluate the options provided.

Assertion:

A body that is a good radiator is also a good absorber of radiation at a given wavelength.

This statement is true. The ability of a body to emit radiation (radiate) is closely linked to its ability to absorb radiation. This relationship is a fundamental aspect of thermal radiation and is supported by experimental observations. In essence, a body that radiates well at a certain wavelength will also absorb well at that same wavelength.

Reason:

According to Kirchhoff's law, the absorptivity of a body is equal to its emissivity at a given wavelength.

This statement is also true. Kirchhoff's law of thermal radiation states that for a body in thermal equilibrium, the emissivity ($\epsilon$) and the absorptivity ($\alpha$) are equal at a given temperature and wavelength:

$\alpha = \epsilon$

This means that if a body has a high emissivity (is a good radiator), it must also have a high absorptivity (is a good absorber) at that particular wavelength.

Evaluation of Options:

Now, let's analyze the provided options.

A) Both assertion and reason are true and reason is the correct explanation of assertion.

This option is correct. Since both the assertion and the reason are true, and the reason accurately explains why the assertion holds, this option is valid.

B) Both assertion and reason are true but reason is not the correct explanation of assertion.

This option is incorrect. While it is true that both statements are accurate, the reason directly explains the assertion according to Kirchhoff's law, making this option invalid.

C) Assertion is true but reason is false.

This option is incorrect. The assertion is true, and the reason is also true (as per Kirchhoff's law), so this option does not hold.

D) Both assertion and reason are false.

This option is also incorrect. As established, both the assertion and the reason are true.

Conclusion:

The correct answer is A, as both the assertion and the reason are true, and the reason provides a valid explanation for the assertion. This relationship between absorptivity and emissivity is a cornerstone of thermodynamics and radiation theory, confirming the interconnectedness of these properties in thermal radiation processes.

Key Concept Recap:

• Kirchhoff's Law: At thermal equilibrium, the absorptivity $\alpha$ equals emissivity $\epsilon$ for a given wavelength, which establishes the direct connection between absorption and emission of radiation.

This understanding is crucial for students preparing for exams like NEET/JEE, as it highlights the fundamental principles governing thermal radiation.