$A$ convex lens forms a real image of an object for its two different positions on a screen. If the height of the image in both cases is $8 \ cm$ and $2 \ cm$,then the height of the object is ........ $cm$.

An object approaches a convergent lens from the left of the lens with a uniform speed $5 \ m/s$ and stops at the focus. The image :-

$A$ thin double-concave lens is made of a very thin transparent material. It can be filled with air or two liquids $L_1$ and $L_2$ having refractive indices $n_1$ and $n_2$ respectively. The lens will converge a parallel beam of light if it is filled with ........

$A$ glass concave lens is placed in a liquid in which it behaves like a convergent lens. If the refractive indices of glass and liquid with respect to air are $_a \mu_g$ and $_a \mu_l$ respectively,then

The diameter of a plano-convex lens is $6 \, cm$ and its thickness at the centre is $3 \, mm$. If the speed of light in the material of the lens is $2 \times 10^8 \, m/s$,the focal length of the lens is.......$cm$.

The focal length of a convex lens is $30 \ cm$ and the size of the image is a quarter of the object,then the object distance is ........ $cm$.