JEE Main · 2019 · 12 Jan 2019 · Shift 2 · Geometrical Optics

In air the biconvex lens forms a sharp real image on the screen. Dropping the whole set-up into water raises the surrounding index, so the lens index relative to its medium falls from about $1.5$ to roughly $\frac{1.5}{1.33}$. By the lens-maker's relation the focal length lengthens sharply $-$ the lens becomes much weaker, its focal length roughly four times the air value. With the object and screen left exactly where they were, the rays no longer converge at the screen, so no sharp image is formed there and the image is lost from the screen.