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What does a light 'filter' do?

  1. May 31, 2010 #1
    I'm doing a question about a laser which emits blue and green light.

    It asks about the light after it has passed through 'a green filter'. What passes through, green or blue light?

    I originally thought green light must pass through, then realised it could mean green light is reflected.
  2. jcsd
  3. May 31, 2010 #2
    A light filter actually stops a particular wavelength of light from passing through it.
    Eg : If there is green filter placed, then green light is stopped and all the rest of the colours are allowed to pass.
  4. May 31, 2010 #3

    Andy Resnick

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    The way you present the question is ambiguous, and certainly can be interpreted to be a green 'pass' filter or a green 'stop' filter.

    Conventionally, saying 'a green filter' means a pass filter- only green passes through the filter.

    The details of what a filter does is dependent on the kind of filter. There are absorptive filters and reflection filters; The mechanism can include absorption, interference, total internal reflection, birefringence, thin film interference, or some combination of those; there are passband filters, notch filters, long pass filters, short pass filters, polarization filters, spatial filters (which are not spectral), etc...
  5. May 31, 2010 #4
    That's basically all the question says. It could possibly be something from the old spec, where the textbook would define exactly what a filter does. I just thought there might be a convention or something.
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