We have two parallel mirrors, which are located at y=0 and y=l in the (x,y) plane. A photon is travelling between the mirrors, up and down along the y-axis. Consider an observer O at rest w.r.t. the mirrors. What's the time (Δt) measure by O for the photon to make a full period. Consider an observer O' which moves along the x-axis with a speed v, which is constant. Assume l' = l. Using Galilean transformations from Newt. mech. , what's the time measured by O' for the photon to make a full period (draw a picture to illustrate the logic). Compare this with the time measured by O. Now use the postulate that c=c'. Compute the period Δt' again and compare it to Δt, writing it as Δt' = γΔt, for some value of γ. Check γ>1. Interpret the results. So for 1, the answer is Δt=2l/c. But I don't know how to do 2 (and as a result of that, 3). I don't know what to draw and the Galilean transformation eludes me. First I thought that you had to draw a triangle, with sides of 0.5vt, l and ct/2. But that accomplishes nothing. Can I get some help please.