Where is the object placed to observe it through a compound microscope?

Rays from the Sun converge at a point $15 \, cm$ in front of a concave mirror. Where should an object be placed so that the size of its image is equal to the size of the object?

$A$ $4.5 \, cm$ needle is placed $12 \, cm$ away from a convex mirror of focal length $15 \, cm$. Give the location of the image and the magnification. Describe what happens to the image as the needle is moved farther from the mirror.

$(a)$ Write any three differences between concave and convex mirror.
$(b)$ Draw ray diagram to show what happens to a ray of light after reflection from a convex mirror,if it
$(i)$ is incident parallel to principal axis.
$(ii)$ incident at the pole.

$(a)$ State the laws of refraction.
$(b)$ $A$ ray of light enters from a medium $A$ into a slab made up of a transparent substance $B$ (as shown in the figure). The refractive indices of medium $A$ and $B$ are $2.42$ and $1.65$ respectively. Complete the path of the ray of light until it emerges out of the slab.

What is a lens?