What is a zoom lens in what is the science behind
plz answer !

The zoom lens is a bi-convex lens

Explanation:

The convex lens has the property of magnifying the image.The bi-convex lens has two concave surfaces.This lens has the property that when parallel rays fall on one of the surfaces, it gets converged on the other side.So, when an object is placed between the focal length and optic center of the lens, it produces a virtual magnified image of the object.The magnification factor is given by the formula

                                 

                              - height of image

                              - height of the object

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