# Optics - Several lenses

by sz0
Tags: lenses, optics
 P: 12 1. The problem statement, all variables and given/known data 1) I have five lenses with f=25 placed at distance 50 from each other, what is the focal length of this system? 2) A light source is placed in the focal point of a lens with f=50, at d=600 a lens with f=400 is placed, at the focal point of the second lens the image is captured, what is the magnification? 2. Relevant equations $\frac{1}{A}$+$\frac{1}{B}$ = $\frac{1}{F}$ is everything, I think... 3. The attempt at a solution 1) I think the light rays will converge 25 after the last lens because for lens 2 and 4: 1/-f + 1/b = 1/f → b=-∞ so the light is parallel with x-axis after these lenses. For lens 1,3,5: 1/∞ + 1/b = 1/f → b=f so the projection is in the middle and for the last lens 25 away. 2) I'm thinking the light is parallel to x-axis after the first lens because 1/f +1/b =1/f → b=∞ and then for the second lens 1/∞ + 1/b = 1/f → image in f, so no magnification has occured. ------ It's not certain both of my solutions are wrong, but probably both are :(. If someone has a link to info about how to calculate lens after lens that would be great, I cant goggle it since I don't know what this sort of problem is called and my book is terrible.no help. I am very happy for any help I can get, so thanks in advance.