$A$ point source of $100$ candela is held $5\,m$ above a sheet of blotting paper which reflects $75\%$ of light incident upon it. The illuminance of the blotting paper is

$A$ ray of light enters from air into water. Which of the following figures shows the dispersion of light?

$A$ ray of light falls on the surface of a spherical glass paperweight,making an angle $\alpha$ with the normal,and is refracted in the medium at an angle $\beta$. What is the angle of deviation of the emergent ray from the direction of the incident ray?

$A$ prism having an apex angle $4^o$ and refractive index $1.5$ is located in front of a vertical plane mirror as shown in the figure. Through what total angle is the ray deviated after reflection from the mirror (in $^o$)?

Which one of the following spherical lenses does not exhibit dispersion? The radii of curvature of the surfaces of the lenses are as given in the diagrams.

How can we change a camera from $F/4$ to $F/5.6$?