Where does an object and its image coincide in a concave mirror?

$(a)$ 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.

Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

Give two uses of a convex lens.

$(a)$ The refractive index of diamond is $2.42$. What is the meaning of this statement?
$(b)$ Redraw the diagram given below in your answer book and complete the path of the ray.
$(c)$ What is the difference between virtual images produced by concave and convex mirrors?
$(d)$ What does the negative sign in the value of magnification produced by a mirror indicate about an image?

Where should the object be placed to obtain a real and diminished image using a concave mirror?