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.

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

 Due to refraction.

Concave mirror and concave lens.

Uses of concave mirror:
(1) As a dentist's head mirror: A circular mirror attached to a band tied to the forehead of the dentist / ENT specialist. A parallel beam of light is made to fall on the concave mirror; this mirror focuses the light beam on a small area of the body.
(2) As a make-up mirror: When a concave mirror is held near the face, an upright and magnified image is seen. Here, our face will be seen magnified.
(3) Concave mirrors are also used as reflectors in torches, head lights in vehicles and search lights to get powerful beams of light.
(4) Concave reflectors are also used in room heaters.
(5) Large concave mirrors are used in solar heaters.
Uses of convex mirror:
(1) Convex mirrors are used as rear-view mirrors in vehicles.
(2) A convex mirror provides a much wider field of view compared to plane mirror.
(3) Convex mirrors are installed on public roads as traffic safety device.

The critical angle for diamond - air interface ( \(\theta _{ c }\) = 24.4°) is very small; therfore once light enters a diamond, it is very likely to undergo total internal reflection inside it: Diamonds faces in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond siutably, multiple total internal reflections can be made to occur.

Light from the star asses through our atmosphere before it reaches our eyes. This light bends (refracts) due to the varying densities and temperature of atmosphere.
Therefore, the light which reaches us appears to come from different points. This gives the impression that stars are twinkling.

The spherical mirror equation is \(\frac { 1 }{ f } =\frac { 1 }{ { u } } +\frac { 1 }{ v } \)
u = distance of object.
v = distance of image.
f = focal length of spherical mirror.

Magnification is defined as the ratio of the height of the image \(\left( { h }_{ i } \right) \) to the height of object \(\left( { h }_{ 0 } \right) \)
Magnification \(=\frac { { h }_{ i } }{ { h }_{ 0 } }\)
1. Real image is negative sign.
2. Virtual image is positive sign
 

R = 2-1, F = \(\frac { r }{ 2 } \)
r = 30 cm (or) 0.3 m
\(F=\frac { 0.3 }{ 2 } \) = 0.15 m or 15 cm
The focal length of convex mirror is = 15 cm

The laws of refraction of light : The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a light of given colour and for the given pair of media. This law is also known as Snell's law of refraction.
 If i is the angle of incidence and r is the angle of refraction, then
\(\frac { \sin { i } }{ \sin { r } } =constant\)
Verification of laws of refraction :

(1) Fix a sheet of white paper on a drawing board using drawing pins.
(2) Place a rectangular glass slab over the sheet in the middle. Draw the outline of the slab with a pencil. Let us name the outline as ABCD. Take four identical pins.
(3) Fix two pins. Say E and F, vertically such that the line joining the pins is inclined to the edge AB.
(4) Look for the images of the pins E and F through the opposite edge. Fix two other pins, say G and H, such that these pins and the images of E and F lie on a straight line.
(5) Remove the pins and the slab.
(6) Join the prints of the pins E and F and let it meet AB at O. Let EF meet AB at O. Similarly, join the prints of the pins G and H and Let it meet HG at O'. Join o and O'. Also produce EF as shown by a dotted line in Figure.
(7) Draw a perpendicular NN' to AB at O and another perpendicular MM' to CD at O'. In this activity, you will note that, the light ray has changed its direction at points O and O'. Note that both the points O and O' lie on surfaces separating two transparent media. The light ray has entered from air to glass and has bent towards the normal that is from a rarer to denser medium.
(8) The light ray has emerged from glass to air that is from a denser medium to a rarer medium. The light here has bent away from the normal. Compare the angle of incidence with the angle of refraction at both refracting surfaces AB and CD.
(9) In Figure, EO is the incident ray OO' the refracted ray and O'H the emergent ray. You may observe that the emergent ray is parallel to the direction of the incident ray.
(10) The extent of bending of the ray of light at the opposite parallel faces AB (air-glass interface) and CD (glass-air-interface) of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray.