A convex lens is thicker in the middle and thinner in the sides. When light from convex lens passes due to the shape of the lens they meet at a focal point and if we keep a piece of paper on that focal point then the the paper will start burning because at that particular point the amount of heat would be the maximum,as in an convex lens the light from all the directions along with their heat meet at the focal point and thus it makes the focal point a place where the level of heat is the highest other than the concave lens where the light along with it's heat move to different directions this can also be said that it is a different and unique feature about convex lens the image formed in an convex lens is said to be an real image as it is inverted.
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Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.
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