Light frequencies and colors

Assuming a flat pane of colored glass, using colored lights...
If you pass red light through red glass, will it be blocked?
How about if you pass red light through a green glass- What will the color/frequency of the light which passes through the glass be? Would you simply add the frequency values together and divide them to get the new color frequency?
 
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We did this experiment in school using coloured plexiglass sheets and a projector. The question posed was "what colour do you get if you place a red and green sheet over a white projector bulb?". Everyone gave the usual answers of what colour red and green paint gives when mixed. Note that this is incorrect.

Red glass only lets red light through. Green glass only lets green light through.

If you shine red light on red glass, then it will pass through. If you shine red light on green glass, none of it will pass through.

To demonstrate this our teacher placed a red sheet on the projector so it filtered the white light down to only red. He then placed a greed sheet on top. Because the only light hitting the green sheet was red, the green sheet completely blocked it showing a black square on the screen.
 

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