hey patelankit ......

a prism is a transparent medium bounded by any no. of surfaces such that the surface facing incident ray and the surface facing the emergent ray are plane and non parallel ......... 

now the prism basically is nothing special but application of snells law .....

but the incident ray can be made incident on the surface at various angles ..... now there occurs a situation when the ray emerges at the same angle as it was incident at ..... this condition corresponds to the following phenomenon:

1. the incident and the emergent angle are equal...

2. the ray inside the prism travels parallel to the base of the prism

3. the deviation of the ray from its original direction is minimum in this case

4. the angle after refraction at incident face inside the prism becomes equal to A/2 , A being the prism angle.....

so if you draw a diagram and work out the snells law at the two faces , along with some geometry you can have the above results.....

also this angle of minimum deviation has got application as for small A and small angles of incidence these results reduce to a relation between refractive index and the angle of deviation , allowing us to determine the refractive index experimentally !!!!!!!!!!!!

..................keep it cooolllll..............

i want to know limiting angle of prism & not minimum angle of deviation