Define the term Angle of Deviation.

Hint: As light travels from one medium to another, its speed varies (for example, from air into the glass of the prism). The light is refracted and enters the new medium at a different angle as a result of the speed shift (Huygens principle). The degree of bending of the light path is determined by the angle made by the incident beam of light with the surface, as well as the ratio of the two media's refractive indices (Snell's law).

Complete answer:
Dispersion is a phenomenon in which the refractive index of various materials (such as glass) changes with the wavelength or colour of the light utilised. As a result, light of different hues is refracted differently and exits the prism at different angles, giving a rainbow appearance. This may be used to split a white light beam into its constituent colour spectrum. Iridescent materials, such as a soap bubble, separate in a similar way. Prisms, unlike diffraction gratings, scatter light across a considerably wider frequency bandwidth, making them ideal for broad-spectrum spectroscopy.
In a prism, the angle of deviation ( $ \delta $ ) decreases as the angle of incidence (i) increases up to a certain point. The minimal deviation point of a prism is the angle of incidence where the angle of deviation in the prism is the smallest, and the angle of deviation is known as the minimum angle of deviation (denoted by $ {\delta _{min}},\;{D_\lambda },{\text{ }}or\;{D_m} $ ).
The angle of minimum deviation is given with the Refractive index as:
 $ {n_{21}} = \dfrac{{\sin \left( {\dfrac{{A + {D_m}}}{2}} \right)}}{{\sin \left( {\dfrac{A}{2}} \right)}} $
This is useful for calculating a material's refractive index. At minimal divergence, a rainbow and halo appear. A narrow prism is also always adjusted to the smallest deviation possible.
The refracted light in the prism is parallel to its base at minimal deviation. To put it another way, the light beam is symmetrical along the prism's axis of symmetry. Refraction angles are also identical, i.e. $ {r_1} = {r_2} $ . In addition, the angle of incidence and the angle of emergence are equal I = e).

Note:
Sometimes We also use
 $ \therefore {D_m} = 2{\sin ^{ - 1}}\left( {n\sin \left( {\dfrac{A}{2}} \right)} \right) - A $
By sending a light beam through a prism of minimal thickness at minimum deviation filled with the material or in a glass prism dipped in it, this is a practical technique to determine the refractive index of a substance (liquid or gas).