$A$ camera lens is set at $f/2$ with an exposure time of $1/100 \ s$. If the lens is set at $f/4$,what will be the correct exposure time?

$A$ 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

Which of the following could not produce a virtual image?

$A$ point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance $E$ at this portion varies with its distance $r$ from the source as:

$A$ container is filled with water $(\mu = 1.33)$ up to a height of $33.25\ cm$. $A$ concave mirror is placed $15\ cm$ above the water level and the image of an object placed at the bottom is formed $25\ cm$ below the water level. The focal length of the mirror is (in $cm$)

The camera lens has an aperture of $f$ and the exposure time is $(1/60) \, s$. What will be the new exposure time if the aperture becomes $1.4 \, f$?