What does a light 'filter' do?

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Discussion Overview

The discussion revolves around the function of a light filter, specifically in the context of a laser emitting blue and green light and the implications of passing this light through a green filter. Participants explore the definitions and types of filters, as well as the ambiguity in the terminology used in the question.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether green or blue light passes through a green filter, initially assuming green light would pass but later considering the possibility of reflection.
  • Another participant asserts that a light filter stops specific wavelengths from passing, suggesting that a green filter would stop green light while allowing other colors to pass.
  • A participant highlights the ambiguity in the question, noting that 'a green filter' could imply either a pass filter or a stop filter, depending on interpretation.
  • Discussion includes various types of filters and their mechanisms, such as absorptive filters, reflection filters, and different filter types like passband and notch filters, indicating the complexity of the topic.
  • One participant speculates that the question may originate from an older specification where the function of filters was clearly defined.

Areas of Agreement / Disagreement

Participants express differing views on the function of a green filter, with no consensus reached on whether it allows green light to pass or stops it. The discussion remains unresolved regarding the interpretation of the term 'green filter.'

Contextual Notes

The discussion highlights the dependence on definitions and the potential for ambiguity in terminology related to light filters. The specific mechanisms of different types of filters are not fully resolved.

Magma828
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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 realized it could mean green light is reflected.
 
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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.
 
Magma828 said:
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 realized it could mean green light is reflected.

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...
 
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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