$A$ photograph of the moon was taken with a telescope. Later on, it was found that a housefly was sitting on the objective lens of the telescope. In the photograph:

$A$ finite size object is placed normal to the principal axis at a distance of $30 \ cm$ from a convex mirror of focal length $30 \ cm$. $A$ plane mirror is now placed in such a way that the image produced by both the mirrors coincide with each other. The distance between the two mirrors is (in $cm$)

When sunlight falls normally on Earth,a luminous flux density (illuminance) of $1.57 \times 10^5 \; lm/m^2$ is produced on Earth. If the distance of Earth from the Sun is $1.5 \times 10^8 \; km$,what is the total luminous flux emitted by the Sun?

In an experiment for estimating the value of focal length of a converging mirror,the image of an object placed at $40\,cm$ from the pole of the mirror is formed at a distance of $120\,cm$ from the pole of the mirror. These distances are measured with a modified scale in which there are $20$ small divisions in $1\,cm$. The value of error in the measurement of the focal length of the mirror is $1/K\,cm$. The value of $K$ is $..........$

$A$ 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.

If the focal length of the objective lens is increased,then the magnifying power of: