A \"floating strawberry\" illusion can be produced by two parabolic mirrors, eac

Question

A "floating strawberry" illusion can be produced by two parabolic mirrors, each with a focal length of 7.5 cm, facing each other so that their centers are 7.5 cm apart (see figure below). If a strawberry is placed on the bottom mirror, an image of the strawberry forms at the small opening at the center of the top mirror. Show that the final image forms at that location and describe its characteristics. A flashlight beam shone on these images has a very startling effect: Even at a glancing angle, the incoming light beam is seemingly reflected off the images of the strawberry! Do you understand why?

Explanation / Answer

As, 1/v + 1/u = 1/f

    distance of first image

=>   1/v = 1/7.5 - 1/7.5

=>   v = infinity    ---------->   distance of first image

   distance of second image

=>    1/v = 1/7.5 - 1/ infinity

=>   v =   7.5   cm         --------------->   distance of second image

=>   Final image of strawberry is formed at the small opening at the centre of the top mirror .

=> Image formed is real .

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