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Homework Help: 2 quick lens concept questions

  1. May 1, 2008 #1
    1. The problem statement, all variables and given/known data

    1. A converging lens is used to focus parallel beams of red light and then later used to focus parallel beams of green light. Which focal length is longer? Explain

    2. A photographer moves closer to his subject & then refocuses the camera. Is the camera lens now closer to or further from the film? Explain

    2. Relevant equations

    3. The attempt at a solution

    I want to say red for the first one. I'm guessing that from snells law green light will converge quicker since it has a higher index of refraction than red light?

    For the 2nd one I want to say further...Since the camera is closer the light coming in converges faster so the new focal point will be further away from the film?
  2. jcsd
  3. May 1, 2008 #2


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    Science Advisor
    Homework Helper

    "Blue Bent Best" - the green light focusses at a shorter distance than red.
    So yes, the effective focal lenght for green light is shorter.

    Correct again.
    A useful rule to remember is that a lens at a distance 'f' focus light at infinity a lens at a distance 2f focuses light from a distance 2f.
  4. May 1, 2008 #3
    thanks for the response!
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