Two convex lenses of focal lengths $f_1$ and $f_2$ form images with magnification $m_1$ and $m_2$,when used individually for an object kept at the same distance from the lenses. Then,$f_1 / f_2$ is

If ${I_1}$ and ${I_2}$ are the sizes of the images for the two positions of the lens in the displacement method,then the size of the object is given by:

$A$ screen is placed $90 \, cm$ from an object. The image of the object is formed on the screen by a convex lens at two different locations separated by $20 \, cm$. If the sizes of the images formed at these positions are $6 \, cm$ and $3 \, cm$,then the height of the object is.....$cm$.

An object $2.4 \ m$ in front of a lens forms a sharp image on a film $12 \ cm$ behind the lens. $A$ glass plate $1 \ cm$ thick,of refractive index $1.50$ is interposed between the lens and the film with its plane faces parallel to the film. At what distance (from the lens) should the object be shifted to be in sharp focus on the film (in $m$)?

When a convex lens is immersed in a liquid of refractive index equal to $80 \%$ of the refractive index of the material of the lens,the focal length of the lens increases by $100 \%$. The refractive index of the liquid is

$A$ convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will