$(a)$ 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.

(N/A) When light travels obliquely from one medium to another,the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.
$(b)$ $A$ negative sign for the image distance $(v)$ in the case of spherical lenses indicates that the image is virtual and erect. It also signifies that the image is formed on the same side of the lens as the object.