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  4. Assertion: The relative velocity of two photons travelling in opposite direction is the velocity of light. Reason: The rest mass of a photon is zero.

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Q. Assertion: The relative velocity of two photons travelling in opposite direction is the velocity of light.
Reason: The rest mass of a photon is zero.

AIIMSAIIMS 1996Dual Nature of Radiation and Matter

Solution:

As the rest mass of a photon is zero, it always moves with the speed of light. And as we know that no material body can ever travel with a speed more than the speed of light in vacuum. Now as the two photons move with speed $c$ of light opposite to each other, their relative velocity is also $c$.