JEE Main 2025 · 29 January · morning · Single correct · +4 / −1
Given below are two statements: one is labelled as Assertion (A) and the 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) Both (A) and (R) are true and (R) is the correct explanation of (A)
- (b) Both (A) and (R) are true but (R) is not the correct explanation of (A)
- (c) (A) is false but (R) is true
- (d) (A) is true but (R) is false
Answer and solution
Answer (c) (A) is false but (R) is true
Solution
Physics sir · 3 steps
Zero mass does not mean zero momentum: a photon carries momentum p = E/c, so the assertion fails.
- Check the assertion. An EM wave carries momentum along with its energy. When energy U is delivered to a surface, the momentum delivered is p = U/c, and this is why light exerts radiation pressure. So (A) is false.
E² = p²c² + m₀²c⁴ ⇒ m₀ = 0 ⇒ p = E/c = h/λ
- Check the reason. A photon has zero rest mass, so (R) is true. Put m₀ = 0 in the relativistic energy relation and the momentum still comes out non-zero, because the photon has energy.
- So. (A) is false and (R) is true, which is option (c).
The idea to keep. For light, momentum comes from energy, not from mass: p = E/c. Radiation pressure is the everyday proof that EM waves carry momentum.
Answer from the NTA final answer key. Solution by PrepWiser.