The definition of angle of prism is, "The angle formed due to two lateral faces of the prism is known as the angle of prism".
My question is if there is an isosceles prism then which angle should be taken as angle of prism,i.e., the two equal angles or the unequal angle? And what will be the answer in-case of a scalene prism?
You have to consider the angle between those two lateral faces, one from which the light enters another from which light leaves.
For an example here, angle A is the angle of prism.
Whether it is isosceles or scalene prism always consider the angle between the lateral sides, where the ray enters and from where it leaves, as the angle of prism.
For just in case, if something like this happens,
Here, $\theta$ is the angle of prism
The more accurate definition of the apex angle ($\Phi$) or the angle of prism is the angle between the surface at which the light enters the prism and the second surface that the light encounters.
This definition should make it clear for an isosceles or scalene prism
Hope this helps.
The angle of prism is the angle between the refracting faces of the prism. You shine your beam on one face and it emerges from the other face. Just measure the angle between those two faces.
Basically, as the answers show, we consider a prism to have infinite extent in the direction opposite the "prism angle" , so it doesn't have to be a triangle. So long as the refracted ray exits the second face, the remainder of the prism could have anywhere from zero (infinite extent) to hundreds of facets.
Obviously this doesn't apply if you are considering the path taken by internal reflections. See "Penta prism" , for example.
