Question

why the angle of incidence must not be less 30 degree while calculating angle of minimum deviation​

Answer 1

In the above given case the angle of incidence and angle of emergence are equal while the angle of the slab for the prism that is being used is 60.

Now we know that the sum of the angles at which the ray is incident or emerging is equal to Sum of two angles of deviation in prism then putting this in the form of formula we get:-

A+D = I+E

Here I is equal to E and the value of it is 30.

A + D = 30 + 30

⇒ 60 + D =60 ⇒ D= 0

This shows that deviation of the angle from incidence is zero.

To know more:

Define angle of deviation and angle of prism