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 :

We need to evaluate the Assertion and Reason about electromagnetic waves.

Assertion (A): "Electromagnetic waves carry energy but not momentum."

Analysis: This is FALSE. Electromagnetic waves carry both energy and momentum. The momentum of an electromagnetic wave is related to its energy by $$p = E/c$$, where $$c$$ is the speed of light. This momentum is responsible for radiation pressure. Experiments such as Nichols radiometer have confirmed that light exerts pressure on surfaces, proving that EM waves carry momentum.

Reason (R): "Mass of a photon is zero."

Analysis: This is TRUE. A photon has zero rest mass. However, it still carries momentum given by $$p = h/\lambda = E/c$$, as described by the relativistic energy-momentum relation $$E^2 = (pc)^2 + (m_0 c^2)^2$$. With $$m_0 = 0$$, this gives $$E = pc$$, confirming that massless photons can carry momentum.

Note: Even though R is true, it does not explain A (which is false). The zero rest mass of a photon does not mean EM waves lack momentum -- in fact, photons carry momentum despite having zero rest mass.

The correct answer is Option 3: (A) is false but (R) is true.