$A$ lens with a focal length of $50 \ cm$ forms an image of a distant object that subtends an angle of $1 \text{ milliradian}$ at the lens. What is the size of the image in $mm$?

$A$ convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black,the image will

In an experiment to measure the focal length $(f)$ of a convex lens,the least counts of the measuring scales for the position of the object $(u)$ and for the position of the image $(v)$ are $\Delta u$ and $\Delta v$,respectively. The error in the measurement of the focal length of the convex lens will be:

$A$ glass convex lens $(\mu_g = 1.5)$ has a focal length of $8\, cm$ in air. What will be its focal length when it is immersed in water (in $, cm$)? $(\mu_w = 1.33)$

$A$ hollow convex lens of glass will behave like a

Define power of a lens,obtain its equation and write its $SI$ unit.