In the case of the displacement method of lenses,the product of magnification in both cases is

$A$ lateral object of height $h_o = 0.5\, cm$ is placed on the optical axis of a bi-convex lens of focal length $f = 80\, cm$,at an object distance $u = -60\, cm$. The image formed is:

An object is moving towards a converging lens on its axis. The image is also found to be moving towards the lens. Then,the object distance $u$ must satisfy:

$A$ thin convex lens of focal length $f$ made of crown glass is immersed in a liquid of refractive index $\mu_l$ $(\mu_l > \mu_c)$,where $\mu_c$ is the refractive index of the crown glass. The convex lens now acts as:

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 $2 \times 10^8 \ m/s$,find the focal length of the lens in $cm$.

An object is placed at a distance of $20 \ cm$ from a convex lens of focal length $10 \ cm$. The image is formed on the other side of the lens at a distance of . . . . . . $cm$.