The dispersive power of the material of a lens of focal length $20 \; cm$ is $0.08$. The longitudinal chromatic aberration of the lens is ...... $cm$.

$A$ ray of light falls on the surface of a spherical glass paperweight,making an angle $\alpha$ with the normal,and is refracted in the medium at an angle $\beta$. What is the angle of deviation of the emergent ray from the direction of the incident ray?

Physical processes are sometimes described visually by lines. Only the following can cross.

An object is placed in front of a convex mirror at a distance of $50 \ cm$. $A$ plane mirror is introduced covering the lower half of the convex mirror. If the distance between the object and the plane mirror is $30 \ cm$,it is found that there is no gap between the images formed by the two mirrors. The radius of the convex mirror is .......$cm$.

Which of the following can form a diminished,virtual,and erect image of your face?

$A$ water drop in air refracts the light ray as: