Which has more luminous efficiency?

$A$ 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?

$A$ 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.....$\%$

$A$ $60 \text{ W}$ bulb is hung over the center of a table of size $4 \text{ m} \times 4 \text{ m}$ at a height of $3 \text{ m}$. The ratio of the intensities of illumination at a point on the center of the edge $(A)$ and at the corner of the table $(B)$ is:

$A$ point object is kept at the first focus of a convex lens. If the lens starts moving towards the right with a constant velocity, the image will

Which of the following phenomena are involved in the formation of a rainbow?