"use geometry"--Redshift derivation? 1. Use geometry to derive z=v/c where c is the speed of light and z = [v(obs)-v(em)]/v(em). 2. Relevant equations None given... Though I am assuming that I am constant. 3. The attempt at a solution I believe it has to do with the Doppler effect which gives Δf = (Δv/c)f. I'm not entirely sure what it means by geometry though; am I looking at the geometry of the changes of light, cause that seems super bizarre to me. I feel so close to getting it but I feel like I'm missing something.