Galilean Telescope

1. Feb 10, 2009

davevresk

1. For a Lab I have set up a "galilean telescope" where the objective lens a convex lens with focal length about 20 cm (F) and a biconcave lens of unknown focal length. I placed the biconcave lens at about 15 cm away from the convex lens as the ocular (so closer than the focal point) and then focused parallel light onto a screen. The distance between the screen and the objective lens was measured to be slightly longer than the focal length of the objective lens (about 22 cm). The question is to find the focal length of the biconcave lens using ray diagrams and the thin lens law.

2. Relevant equations
Thin lens law: 1/f=(n-1)(1/r1 - 1/r2) where n is the refractive index of the lens.

3. The attempt at a solution
I have drawn a ray diagram but it doesn't seem to match what is in the text book. The book only shows diagrams where light enters the biconcave lens and come out parallel, but that wouldn't focus the light right? So it must not come out parallel in this case. So in my diagram the incident (parallel) light is bent inwards by the objective lens to a focal point that is beyond the ocular lens, so before it is actually focused, it goes through the ocular (biconcave) lens at which point it is bent to an angle that doesn't converge quite as fast so it focuses at point beyond the focal point of the objective light. I don't really know how this helps me because the light entering the ocular is not parallel and so it doesn't converge on the lens focal point (or does it). Also we are not given the radius of the lenses or the index of refraction in the lab manual, so I dont really see how thin lens law really helps at all.

2. Feb 11, 2009

davevresk

nm, i think i figured it out