A prism PQR (see figure) is a wedge-shaped body made from refracting medium bounded by two plane faces inclined to each other at an angle.
![](https://www.sarthaks.com/?qa=blob&qa_blobid=15089351220350666043)
When a ray of light passes through a prism, it bends towards the base of prism and the angle made by the emergent ray with the incident ray is called angle of deviation. As the angle of incidence increases, the angle of deviation decreases to minimum δm. The angle of minimum deviation is given as
![](https://www.sarthaks.com/?qa=blob&qa_blobid=1672736503354373062)
where A is the angle of the prism, μ is the refractive index. For an equilateral prism, A = 60°.
Refractive index of the prism (when kept in air) is 1.6. Thus, the angle of minimum duration in air is [using Eq. (1)] given by
![](https://www.sarthaks.com/?qa=blob&qa_blobid=12363607679243269236)
When the prism is immersed in a medium of refractive index 4√2/5 then by using Eq. (1), we get
![](https://www.sarthaks.com/?qa=blob&qa_blobid=11222628568009019743)
Therefore, the angle of minimum deviation of the prism, when it is kept in medium of refractive index 4√2/5, is 30°.