|MadSci Network: Physics|
The answer is: there is no green light going in the filter, so there is no green light coming out! Filters do not change the color of light, but separate a combination of colors by absorbing some and transmitting some - a green filter will absorb red and blue light, while green light passes right through. Filters are also not quite perfect, unless you spend over $100, so a typical green filter will still transmit some red light. You could try and measure how much red light is absorbed by the filter, using a photographic light meter to measure the brightness of the laser spot on a white surface, then measure again with the filter in the beam; perhaps 50% or less will get through. Lasers produce only single colors, a single "line" of the rainbow. The laser beam's color is determined by the materials inside (molecules and atoms charged up and vibrating). Some larger lasers can produce more than one "line" at a time (for example light show lasers). Lasers do NOT produce a full spectrum of white light. Since there is no green light produced by the laser, there is no green light to get through the filter. Remember, filters don't change the color of light, they separate combinations of colors (white light is all the colors combined).
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