The dispersive power of the material of a lens of focal length $20 \; cm$ is $0.08$. The longitudinal chromatic aberration of the lens is ...... $cm$.

$A$ lamp is hanging at a height of $4\,m$ above a table. The lamp is lowered by $1\,m$. The percentage increase in illuminance will be.....$\%$

$A$ convex mirror of focal length $15 \ cm$ and a concave mirror of focal length $10 \ cm$ are placed facing each other at a distance of $40 \ cm$ apart. $A$ point object is placed between the mirrors on their common axis at a distance of $15 \ cm$ from the concave mirror. The position of the image formed by reflection at the convex mirror will be at a distance of ..... $cm$ from it.

An electric bulb illuminates a plane surface. The intensity of illumination on the surface at a point $2 \ m$ away from the bulb is $5 \times 10^{-4} \ phot$ $(lumen/cm^2)$. The line joining the bulb to the point makes an angle of $60^{\circ}$ with the normal to the surface. The intensity of the bulb in candela is

Two blocks each of mass $m$ lie on a smooth table. They are attached to two other masses as shown in the figure. The pulleys and strings are light. An object $O$ is kept at rest on the table. The sides $AB$ and $CD$ of the two blocks are made reflecting. The acceleration of the two images formed in those two reflecting surfaces with respect to each other is:

Assertion : $A$ concave mirror and convex lens both have the same focal length in air. When they are submerged in water,they will have same focal length.
Reason : The refractive index of water is smaller than the refractive index of air.