What is the power of a convex lens with a focal length of $50 \ cm$?

For which type of mirror will the image always be erect,regardless of the distance at which the object is placed?

$A$ pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent,if instead of water we use liquids like kerosene or turpentine? Support your answer with reason.

$A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from the lens. Where should the needle be placed in front of the convex lens so that this image is of the same size as the object? Also, find the power of the lens.

$A$ concave mirror can be used to obtain a parallel beam of light from a small lamp.

How will you decide whether a given piece of glass is a convex lens,concave lens,or a plane glass plate?