$A$ hollow lens is made of thin glass and in the shape of a double concave lens. It can be filled with air,water of refractive index $1.33$,or $CS_2$ of refractive index $1.6$. It will act as a diverging lens,if it is

$A$ thin convex lens is made of a material of refractive index $1.6$. An object is kept at a distance of $u$ from the lens on the principal axis as shown in the figure. The radii of curvature of the surfaces are $10 \, cm$ and $5 \, cm$. Now,the lens is reversed such that the face having radius of curvature $5 \, cm$ lies close to the object. The difference in image position as obtained for both the cases is equal to ......$u$.

$A$ convex lens is made of glass $(\mu_g = 1.5)$ and has a focal length of $4 \, cm$ in air. If it is immersed in water $(\mu_w = 1.33)$,what will be its new focal length in $cm$?

$A$ convex lens is used to form an image of an object on a screen. If the upper half of the lens is blackened,so that it becomes opaque,then

$A$ 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 $cm$)

$A$ convex lens of glass $(\mu_g = 1.45)$ has a focal length $f_a$ in air. The lens is immersed in a liquid of refractive index $(\mu_l) = 1.3$. The ratio of the $f_l / f_a$ is