What does a light 'filter' do?

[Magma828]

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.

 

[gullapalli]

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.

 

[Andy Resnick]

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...

 

[Magma828]

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.