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

Red Light?

  1. Jun 9, 2008 #1


    User Avatar

    Why is it that red light can be used in darkrooms when developing film?
    What is so special about it that it doesnt damage photographic film?
    I figure red light high wavelength, thus less intensity?
  2. jcsd
  3. Jun 10, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Red photons are less energetic and so materials are less sensitive to them.
    Actually you can only handle photographic paper under red light, film is generally handled in complete darkness - otherwise it wouldn't be sensitive to the red light in the picture.

    - there are some special 'blue sensitive' only films that can be handled in red light for specialist applications
  4. Jun 10, 2008 #3
    Intensity is related to (the square of) the amplitude of the light wave. Energy is related (inversely) to the wavelength of the light wave.

    Just some minute details but very important.

    Also, you may be interested to know that in semiconductor manufacturing, it is common to use a photoactive polymer matrix for the purpose of micro-patterning. These compounds are sensitive to UV and deep-UV light. So in these rooms, the lighting is yellow. Same idea as the previous post.
  5. Jun 10, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    Just to clear up an important point.
    Wavelength is related to the energy of an individual photon.
    intensity is the total energy/power of a whole bunch of them.
    So red or infrared light has low energy photons but an infrared cutting laser has very high intensity because it puts a lot of them together.

    The very important point for chemical reactions, like those in film, is that a single photon has to knock out a single electron - if the single photon doesn't have enough energy then no matter how many of them you supply they still won't cause the reaction.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook