Find the focal length of a lens of power $-2.0 \, D$. What type of lens is this?

The image formed by a concave mirror is observed to be virtual,erect and larger than the object. Where should be the position of the object?

An object $5 \,cm$ in length is held $25 \,cm$ away from a converging lens of focal length $10 \,cm$. Find the position,size,and the nature of the image formed.

An object $5.0\, cm$ in length is placed at a distance of $20\, cm$ in front of a convex mirror of radius of curvature $30\, cm$. Find the position of the image,its nature and size.

An object of size $7.0\, cm$ is placed at $27\, cm$ in front of a concave mirror of focal length $18\, cm$. At what distance from the mirror should a screen be placed,so that a sharp focussed image can be obtained? Find the size and the nature of the image.

The magnification produced by a plane mirror is $+1$. What does this mean?