1.

Assertion : The relativevelocity of two photons travellingin opposite directions is C. Reason :The rest mass of a photon is zero .

Answer»

If both the ASSERTION and reason are true statement andreason is correct explanation of the assertion .
If both the assertion and reason are true statement but reason is not a correct explanation of the assertion .
If the assertion is true but the reason is a FALSE statement.
If both assertion and reason are false statements.

Solution :Very high velocitiescan not be added directly as in classical mechanics because this would LEAD to a violatio of the POSTULATES ofrelatively . Such velocities are added in accordance with Lorentz transformations. It can be proved that any velocity added to the velocity of light gives velocity of light .
RELATIVISTIC mass of photon -
The relativistic momentum of photon is`p=(m_(0)V)/(sqrt(1-((v^(2))/(c^(2))))`
But`p=(h)/(lambda)`
` :. (h)/( lambda)=(m_(0)V)/(sqrt(1-((v^(2))/(c^(2))))`
`m_(0)=(h)/(v lambda)sqrt(1-((v^(2))/(c^(2)))`
For a photon v=c , then , `m_(0)=0`


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