Which of the following materials has maximum optical density?

$A$ plane mirror produces a magnification of

$(a)$ What is meant by refraction of light?
$(b)$ If on applying Cartesian sign convention for spherical lenses the image distance obtained is negative,state the significance of the negative sign.

$A$ ray of light $AB$ parallel to the principal axis is incident on a convex lens as shown in the figure. Copy the figure.
$(i)$ Draw the refracted ray.
$(ii)$ Name the point through which the refracted ray passes on the principal axis.

The ray diagram given below illustrates the experimental setup for the determination of the focal length of a converging lens using a plane mirror.
$(i)$ State the magnification of the image formed.
$(ii)$ Write two characteristics of the image formed.
$(iii)$ What is the name given to the distance between the object and the optical centre of the lens in the given diagram?

You are given water,mustard oil,glycerine,and kerosene. In which of these media would a ray of light incident obliquely at the same angle bend the most?