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.

Periscope:-
(i) Periscope is an instrument used for viewing bodies or ships, which are over and around another body or a submarine.
Working:-
(i) It is based on the principle of the law of reflection of light
(ii) It consists of a long outer case and inside this case mirrors or prisms are kept at each end,  inclined at an angle of 45^o.
(iii) Light coming from the distant body falls on the mirror at the top end of the periscope and gets reflected vertically downward.
(iv) This light is reflected again by the second mirror kept at the bottom so as to travel horizontally and reach the eye of the observer.
(v) In some complex periscopes, optic fibre is used instead of mirrors for obtaining a higher resolution.
(vi) The distance between the mirrors also varies depending on the purpose of using the periscope.

Reflection of light:
i) The bouncing back of the light rays as they fall on the smooth, shiny and polished surface is called reflection.
ii) Based on the nature of the surface, reflection can be classified into two types.
i) Regular reflection
ii) Irregular reflection
Regular Reflection :
1. When a beam of light falls on a smooth surface, it gets reflected. This reflection is called regular reflection.
2. After reflection, the reflected rays will be parallel to each other.
3. The angle of incidence and angle of reflection of each ray will be equal.
4. The law of reflection is obeyed in this reflection and thus a clear image is formed,
5. It is also known as specular reflection.
6. Example: Reflection of light by a plane mirror.
Irregular Reflection :
1. In a body having a rough or irregular surface each region of the surface is inclined at different angles.
2. When light falls on such a rough surface, the light rays are reflected at different angles. This reflection is called irregular reflection.
3. The angle of incidence and the angle of reflection each ray are not equal.
4. The law of reflection is not obeyed in this reflection and thus the image is not clear.
5. It is also known as diffused reflection.
6. Example: Reflection of light from a wall.

Images formed by a concave mirror.

Speed of light in air (C) = 3 x 10⁸ ms^-1
Refractive index (μ) = 1.5
Refractive index (μ) = \(\frac { Speed\ of\ light\ in\ air(c) }{ Speed\ of\ light\ in\ medium(v) } \)
Speed of light in the medium (v) = \(\frac { Speed\ of\ light\ in\ air(c) }{ Refractive\ index(\mu ) } \)
V = \(1.5 = \frac { 3 \times { 10 }^{ 8 } }{ v } = \frac { 3 \times { 10 }^{ 8 } }{ 1.5 }\)
v = 2 x 10⁸ ms^-1
Speed of light in the medium (v) = 2 x 10⁸ ms^-1