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

In a grease spot photometer,light from a lamp with a dirty chimney is exactly balanced by a point source at a distance of $10 \, cm$ from the grease spot. On cleaning the chimney,the point source is moved by $2 \, cm$ to obtain balance again. The percentage of light absorbed by the dirty chimney is nearly......$\%$

$A$ man wishing to get a picture of a Zebra photographed a white donkey after fitting a glass with black streaks onto the objective of his camera.

$A$ cube of side $2\, m$ is placed in front of a concave mirror of focal length $1\, m$ with its face $P$ at a distance of $3\, m$ and face $Q$ at a distance of $5\, m$ from the mirror. The distance between the images of face $P$ and $Q$ and the heights of the images of $P$ and $Q$ are:

$A$ concave mirror is placed at the bottom of an empty tank with its face upwards and axis vertical. When sunlight falls normally on the mirror,it is focused at a distance of $32 \, cm$ from the mirror. If the tank is filled with water $\left( \mu = \frac{4}{3} \right)$ up to a height of $20 \, cm$,then the sunlight will now be focused at:

If $\hat{n}_1$ is the unit vector along the incident ray,$\hat{n}_2$ is the unit vector along the normal,and $\hat{n}_3$ is the unit vector along the reflected ray,then which of the following must be true?