$A$ plane wave passes through a convex lens. The geometrical shape of the wavefront that emerges is

An object is moving towards a converging lens on its axis. The image is also found to be moving towards the lens. Then,the object distance $u$ must satisfy:

The focal length of a lens is greatest for which colour?

$A$ glass convex lens has a refractive index of $1.55$ with both faces having the same radius of curvature. What is the radius of curvature required if the focal length is to be $20 \,cm$ (in $\,cm$)?

An object is placed on the axis of a convex lens. Its image is formed $80 \, cm$ from the object. The magnification is $3$. The focal length of the lens is.....$cm$

$A$ convex lens of focal length $f$ $m$ forms a real,inverted image twice the size of the object. The object distance from the lens in metres is: