We know that light has momentum, but doesn't have mass. So if light reflects of say a mirror. The change in momentum is zero. If it is zero then we have a problem. ΔP = ΔFT ΔT = ΔP / F ΔT = 0 / F ΔT = 0 This shows that light is instantaneous because the time it takes for light to bounce of something is zero. Is there something I am doing wrong here? And if I am wrong and there is a change in momentum then wouldn't light create a force when it reflects?