Refractive index of the medium is the ratio of the speed of light in the air to the speed of light in that particular medium.
μ = \(\frac { Speed\ of\ light\ in\ air\ (c) }{ Speed\ of\ light\ in\ the\ medium\ (v) } \)

Applications of a concave mirror:
i) Used as shaving mirror as they provides magnified image
ii) Used in torches, search lights as they direct the light to a long distance.
Applications of a convex mirror:
i) Used in vehicles as rear view mirror
ii) Used on roads where there are sharp curves and turns

If the curved mirror is a part of a sphere, then it is called a spherical mirror.

i) Splitting of white light into its seven constituent colours on passing through a transparent medium is known as dispersion of light.
ii) Dispersion occurs because light of different colours present in white light have different wavelength and they travel at different speeds in a medium.
iii) The refraction of a light ray in a medium depends on its speed.
iv) As each coloured lights has a different speed, the constituent coloured lights are refracted at different extents inside the prism.
v) Refraction of a light ray is inversely proportional to its wavelength.
vi) Thus, the red coloured light which has a large wavelength is deviated less while the violet coloured light which has a short wavelength is deviated more.
vii) Formation of rainbow is an example of dispersion of white light.

Radius of curvature = 20 cm
Focal length (f) =\(\frac { Radius\quad of\quad curvat }{ 2 } \)
=\(\frac { R }{ 2 } =\frac { 20 }{ 2 } \)=10 cm

_wμ_g = \(\frac { Refractive\ index\ of\ glass }{ Refractive\ index\ of\ water } \)
\(=\frac { \frac { 3 }{ 2 } }{ \frac { 4 }{ 3 } } =\frac { 9 }{ 8 } \)=1.125

Angle of inclination = 90°
Number of images for=\(\frac { { 360 }^{ 0 } }{ \theta } -1=\frac { { 360 }^{ 0 } }{ { 90 }^{ 0 } }\)-1=4-1=3