How many centres of curvature does a lens have?

What is the focal length of a plane mirror?

An object is situated at $8 \, cm$ from a convex lens of focal length $10 \, cm$. Find the position and nature of the image. Draw a ray diagram to illustrate the formation of the image (not to scale).

The size of an image of an object formed by a mirror with a focal length of $20\, cm$ is observed to be reduced to $1/3^{rd}$ of its size. At what distance is the object placed from the mirror? What is the nature of the image and the mirror?

$(i)$ Define a real image of an object.
$(ii)$ Name the mirror that:
$(a)$ can give a real as well as a virtual image of an object.
$(b)$ will always give a virtual image of the same size as the object.
$(c)$ will always give a virtual and diminished image of an object.
$(d)$ is used by a doctor in examining teeth.
$(iii)$ With the help of a ray diagram,explain the use of a concave mirror as a solar concentrator.

Zaheen uses a convex lens to make sunlight burn a hole in a piece of card. Zaheen changes the lens in the holder for another lens with twice the power. What will she have to do to focus the sunlight on the card?