Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mirrors, light and camera

  1. Apr 28, 2006 #1
    As far as I understand, the film inside a photo camera is positioned in such a way that every light ray that hits some spot X on the film comes from a single source on the object being photographed. Here's what I mean:


    The various light rays from the top of the pencil, no matter where or at what angle they hit the lens, will be focused to one point on the film.

    If so, then I have two questions:

    1. What about interference effects between the rays of light? Since they all hit the same point on the film, and come from the same source, it seems they will be coherent and should produce interferene at that point.
    Yet we don't see that when taking photographs.

    2. What about when the source of the rays is not an object (pencil, etc.) but a mirror? In that case, even if the rays come from the same point on the mirror, they are still of different colors (frequencies) since they are reflections of light rays entering the mirror. And yet, all these various frequencies from the same point in the mirror will hit one single spot on the film. Shouldn't this produce a mix of colors on the resulting photograph, instead of a picture of reflection that we expect?

    Thanks - Brian
  2. jcsd
  3. Apr 28, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    1. Each ray has a different optical path length, so they will not arive at the lens or image at the same time. Thus there will not be any interference.

    2. Everything we see is a superpostion of all frequencys present. This should not be remarkable.
  4. Apr 29, 2006 #3
    The rays that will arrive at the image at the same time will be coherent (since they are coming from the same source), thus there should be interference.

    Re-read my post above ...The following image shows the situation I have in mind: http://i2.tinypic.com/wsql2r.jpg

    The green and blue rays are reflected off a mirror, at the same point, and then enter the lens and are focused onto a single point A [this will happen for every ray coming from the same point on the mirror]. As a result, any photodector/film/etc. on point A should detect a mix of all colors [from rays coming from all angles], yet in reality we only see one color from one ray.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook