1. The problem statement, all variables and given/known data A He-Ne laser emits red light of the wave length lambda =632.8 nm with a beam diameter of 2.0 mm and a power output of 1.0 mW. (a) What is the intensity of this laser beam? (b) What is the magnitude of the momentum of each photon? (c) How many photons does this laser emit each second? (d) How many photons are there in the 1 m long laser beam? 2. Relevant equations I = P/A N = P/E P= h/lambda 3. The attempt at a solution (a) I = P/A Hence , I = 0.001 / 4*pi*(0.001) = 79.577 W/m (b) P = h / lambda = 1.047 *10^-27 kg.m/s (c) E = h*lambda/c = 1.398 * 10^-48 J N = P/E = 7.15 * 10^44 photons / second (d) I am not sure if the 1 m here is referring to the wavelength or the length - can anyone explain it to me ? Thanks in advance.