(a) In relativity, the relative speed of two objects (vrel ​) moving in the same direction with speeds u and v is given byvrel​=1−c2uv​u−v​…(1) As the photons are moving with the speed of light, u=c and v=c. On substituting the values of u and v in equation (1), we get:vrel ​=0 Thus, relative velocity of a photon with respect to another photon will be 0 , when they are going in the same direction.(b) In relativity, relative speed of two objects moving in opposite directions with speeds u and v is given byvrel​=1+c2uv​u+v​…(2) We know that a photon travels with the speed of light. Therefore, u=c and v=c On substituting the values of u and v in equation (2), we get:vrel​=c Thus, the relative velocity of a photon with respect to another photon will be equal to the speed of light when they are going in opposite directions.