Angle of minimum deviation is?

Hint: In order to answer this question, as we know that the minimum deviation is related with the prism, first we will explain the minimum deviation occurs in the prism and then we will also discuss the angle of minimum deviation.


Complete step by step answer:
The angle of deviation becomes, minimum for a particular angle of incidence of the incident ray on a prism. The angle of least deviation is the smallest value of the angle of deviation. The angle of deviation through a triangular prism is defined as the angle between the incident ray and the emerging ray \[(angle\;\,\delta )\] .
Of course, this is the angle of deviation's smallest value. The angle of incidence and the angle of appearance of the ray are equal at the lowest deviation angle.
The angle of minimum deviation is related with the Refractive index as:
${n_{21}} = \dfrac{{\sin (\dfrac{{A + {D_m}}}{2})}}{{\sin (\dfrac{A}{2})}}$
This is useful to calculate the refractive index of a material. Rainbow and halo occur at minimum deviation. Also, a thin prism is always set at minimum deviation.
In minimum deviation, the refracted ray in the prism is parallel to its base. In other words, the light ray is symmetrical about the axis of symmetry of the prism. Also, the angles of refractions are equal i.e. \[{r_1}\; = \;{r_2}\] . And, the angle of incidence and angle of emergence equal each other $(i = e)$ .

Note: The minimum angle of deviation for a prism is determined by the prism's material, its apex and cut angles, the angle of incidence at which the light ray hits the prism, and the wavelength of light used.