The sun subtends an angle of $(1/2)^o$ at the surface of the earth. $A$ converging lens of focal length $100\, cm$ is used to obtain an image of the sun on a screen. The diameter of the image formed is about.....$mm$.

The optical power of a thin glass lens (refractive index $1.5$) in air is $-5 \ D$. What will be its optical power in a liquid with a refractive index of $1.6$ (in $D$)?

$A$ thin converging lens of focal length $25 \,cm$ forms a sharp image of an object on a screen placed at a distance of $75 \,cm$ from the lens. Later, the screen is moved closer to the lens by a distance of $25 \,cm$. The distance through which the object is to be shifted so that its image on the screen is sharp again is

$A$ convex lens made of glass (refractive index $= 1.5$) has a focal length of $24 \ cm$ in air. When it is totally immersed in water (refractive index $= 1.33$),its focal length changes to: (in $cm$)

Calculate the focal length of the given lens in $cm$,if the magnification is $-0.5$.

The relation between $n_1$ and $n_2$,if the behavior of light rays is as shown in the figure,is: