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physics-james
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When you put a convex and then concave lens in front of a light source, the light will be parallel but narrower than when it came in such as in a laser beam expander/compressor. Using a pen laser and a convex and concave lens, is it possible to focus the beam by putting a convex then a concave lens in front of it using this method? Taking this to the extreme, would it allow you to create a laser from a normal flashlight with enough lens?