# A convex lens placed between an object and screen forms a sharp image of the object on screen. A glass slab of refractive index n is inserted between object and lens. In order to obtain the sharp image on the screen again the lens has to be displaced by a distance x. What is the thickness of the slab? I solved it in the following way: Let t be the thickness of the slab. Shift in position of image due to the insertion of glass slab = [t(n-1)]n i.e. x = [t(n-1)]n t = (nx)/(n – 1) But the answer given in my book is n/x.