1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Thick lenses

  1. Oct 15, 2006 #1
    Here's the problem:

    Here's what I understand:

    I know what a biconcave lens looks like, and how it behaves (for the most part). Of course I know that the index of refraction is ~1.00. And since the radii are equal that makes the equation somewhat easier. The thick lens equation is:
    (Sorry I am not yet familiar with LaTex, so just *imagine* that (1/R1)-(1/R2) isn't there).

    What I don't understand:

    Where is the image diverging from? "Parallel rays from the central axis convirge into a reflected and transmitted image"..does this just mean where the rays convirge?

    Any guidance would be appreciated. Thanks as always!
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Oct 15, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    So, can we reduce the equation to

    1/f = d/2R^2

    For parallel incoming rays the "object" is at infinity and the reflected image is a point. For spherical reflectors this point is half a radius from the surface. I'm not sure I follow the sign convention for the formula, and it's not immediately clear to me how the "transmitted image" will appear. A biconcave lens would form a virtual image, and with thin lenses that would mean a negative focal length. I know that not all sign conventions are the same, so maybe all is OK. I can see how the "transmitted image" could mean the virtual image of the diverging lens. It's not obvious that d has no effect on the position of the virtual image, but maybe that is the point of the problem
    Last edited by a moderator: Apr 22, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook