Consider the head-on collision of two photons, 1 and 2 , of energies E 1

Consider the head-on collision of two photons, γ₁ and γ₂, of energies E₁ > E₂ respectively. If γ₁ is produced by a LASER of wavelength λ=690 nm, what is the minimum value of E2 to produce a positron–electron pair? Compute the velocity of the CM system at threshold as 1 - β. Calculate the mass of the two photons if they move in the same direction.