The beam of light must be passing through the optical centre. The reason being that the middle part of a lens is like a thin glass slab, so no deviation and very slight lateral displacement of the incident ray.

(i) When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours.
(ii) This phenomenon is called as 'dispersion of light'.
(iii) The band of colours is termed as spectrum. This spectrum consists of following colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. These colours are represented by the acronym "VIBGYOR".
(iv ) When white light is refracted by a transparent medium. Different coloured lights are bent through different angles. That is the angle of refraction is different for different colours.
(v) Angle of refraction is the smallest for red and the highest for violet.
(vi) From Snell's law, we know that the angle of refraction is determined in terms of the refractive index of the medium.
(vii) Hence, the refractive index of the medium is different for different coloured lights.

According to optical property, it is classified into two types.
(i) Refracting telescope
(ii Reflecting telescope
(i) Refracting telescope: Lenses are used in this kind of telescopes. Galilean telescope, Keplerian telescope, Achromatic refractors, Apochromatic refractors are some Refracting telescope.
(ii) Reflecting telescope: Parabolic mirrors are used in these kind telescopes. Gregorian, Newtonian, Cassegrain telescope, Ritchey- Chretien telescope are some Reflecting telescopes. According to the things which are observed, Astronomical Telescope and Terrestrial Telescopes are the two major types of telescope.

(i) When a parallel beam of monochromatic (single coloured ) light passes through a gas or liquid or transparent solid, a part of light rays are scattered. This scattering is called Raman scattering (inelastic scattering).
(ii) Raman Scattering is defined as "The interaction of light ray with the particles of pure liquids or transparent solids, which leads to a change in wavelength or frequency".
(iii) The scattered light contains some additional frequencies (or wavelengths) other than that of incident frequency (or wavelength). This is known as Raman scattering or Raman Effect.
(iv) The modified lines are called 'Raman lines' and the unmodified lines are called as 'Rayleigh lines'. The lines having frequencies lower than the incident frequency is called stokes lines and the lines having frequencies higher than the incident frequency are called Anti-stokes lines.