JEE Main 2025 Jan 29 Shift 1, Physics Q20: Electromagnetic Waves
Given below are two statements : one is labelled as Assertion (A) and other is labelled as Reason (R).
Assertion (A) : Electromagnetic waves carry energy but not momentum.
Reason (R) : Mass of a photon is zero.
In the light of the above statements, choose the most appropriate answer from the options given below :
- A
(A) is true but (R) is false.
- B
(A) is false but (R) is true.
- C
Both (A) and (R) are true but (R) is not the correct explanation of (A).
- D
Both (A) and (R) are true and (R) is the correct explanation of (A).
Electromagnetic waves do carry momentum: the momentum density is where is the Poynting vector, and the relation holds for a photon. The reality of radiation pressure (demonstrated by the deflection of comet tails and laboratory experiments) confirms this. Hence Assertion is false.
The rest mass of a photon is indeed zero, so Reason is true.
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