Question

why we dont obtain angke of emergence when angke of prism is very small

Answer 1

When you reduce the angle of incidence beyond the minimum angle of incidence (I don't think it is right to call it the minimum angle of incidence ,it must be rightly called angle of incidence at minimum deviation ) then nothing very strange gonna happen the angle of deviation will increase and the angle of emergence will increase .

This graph shows the variation of angle of deviation with angle of incidence when angle of deviation is less than angle of minimum deviation for the given angle of incidence then angle of deviation decrease with increase in angle of incidence , when angle of deviation is more than angle of minimum deviation then it increases with angle of incidence .