the focal length of a concave lens is 30 centimetre at what distance should the object be placed from the lens so that it forms an image at 20 cm from the lens
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A convex lens and concave lens with focal lengths of 30 cm and 20 cm are placed 10 cm apart. What is the distance of the source of light at which it must be placed for this system to give a parallel beam of rays?
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