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 ...... $cm$.

Which of the following is $NOT$ involved in the formation of a secondary rainbow?

In the diagram shown,all the velocities are given with respect to the earth. What is the relative velocity of the image in mirror $(1)$ with respect to the image in mirror $(2)$? The mirror $(1)$ forms an angle $\beta$ with the vertical.

$A$ lamp rated at $100 \, cd$ hangs over the middle of a round table with diameter $3 \, m$ at a height of $2 \, m$. It is replaced by a lamp of $25 \, cd$ and the distance to the table is changed so that the illumination at the centre of the table remains as before. The illumination at the edge of the table becomes $X$ times the original. Then $X$ is

Two plane mirrors of length $L$ are separated by a distance $L$,and a man $M_2$ is standing at a distance $L$ from the connecting line of the mirrors,as shown in the figure. $A$ man $M_1$ is walking in a straight line at a distance $2L$ parallel to the mirrors at a speed $u$. Then,the man $M_2$ at $O$ will be able to see the image of $M_1$ for a total time of:

$A$ diverging lens of focal length $10 \, cm$ is placed $10 \, cm$ in front of a plane mirror as shown in the figure. Light from a very far away source falls on the lens. The final image is at a distance