deviation angle of prism

The exact solution of the deviation in a prism is complex and is rarely used.

However, for a thin prism, there is an approximate solution:

d = (n-1).A
where d is the deviation angle
A is the apex angle of the prism
n is the refractive index of the prism material

You may refer to the following web-page for the analysis:

http://www.tutorvista.com/content/physics/physics-iv/optics/prism.php