Question

Assertion: EM radiation when falls on a surface exerts pressure. Reason: Radiation has no rest mass.

• A. A and R both are correct and R is correct explanation of A
• B. A and R both are correct but R is not correct explanation of A
• C. A is correct but R is incorrect
• D. A is incorrect but R is correct

Hint:

Momentum of a photon: \[ p = \frac{E}{c} \] Radiation pressure arises due to momentum transfer of photons.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Electromagnetic (EM) radiation consists of photons.
When these photons strike a surface, they transfer their momentum to it.
According to Newton's Second Law, a change in momentum over time manifests as a force, which distributed over an area creates radiation pressure.
Step 2: Key Formula or Approach:
The relationship for radiation pressure is $P = \frac{I}{c}$ (for complete absorption) and momentum is $p = \frac{h}{\lambda}$.
Step 3: Detailed Explanation:
The Assertion is true: EM radiation definitely exerts pressure on surfaces.
This is a well-established phenomenon, responsible for the tails of comets pointing away from the sun.
The Reason states that radiation (photons) has no rest mass. This is also factually true according to the theory of relativity; the rest mass of a photon is strictly zero.
However, does having "zero rest mass" EXPLAIN why it exerts pressure? No.
The reason EM radiation exerts pressure is because it carries momentum ($p = \frac{E}{c} = \frac{h}{\lambda}$), not because its rest mass is zero.
Having no rest mass is a characteristic property of photons, but it isn't the causative reason for the momentum transfer and resulting pressure.
Therefore, R is not the correct explanation for A.
Step 4: Final Answer:
Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.