When a light ray travels from denser medium to rarer medium it bends away from the normal.

When a ray of light travels from rarer medium to the denser medium it bent towards the normal.

When the light rays coming from an object do not actually meet, but appear to meet when produced backwards that image will be virtual. It is erect and cannot be caught on the screen.

If the light rays coming from an object actually meet, after reflection, the image formed will be a real image and it is inverted. The real image can be produced on a screen.

Convex mirrors are preferred because they always give an erect image. They have a wider field view as they are curved outwards.

Refractive index \(\mu \) of diamond = The refractive index is linked to the relative speed of propagation of light in different media. Light propagates with different speeds in different media. (i.e.,) light travels faster in vacuum than in medium.
When light travels from air to diamond the refractive index of the diamond with respect to air.
\(The\ refractive\ index\ \mu =\frac { Speed\ of\ light\ in\ air }{ Speed\ of\ light\ in\ medium } \\ 2.42=\frac { Speed\ of\ light\ in\ air }{ Speed\ of\ light\ in\ diamond } \)
This means the speed of light in air is 2.41 times the speed of light in diamond. The speed of light in diamond is lesser than speed of light in air.

When a ray of light passes from air into glass the angle of refraction is lesser than the angle of incidence.

 
A ray passing through the principal focus of a concave mirror after reflection will emerge parallel to the principal axis.

Concave mirror and concave lens.

  Water, glass.