20° is quite a small angle. If you are also sending light into the prism at a small angle (say ≤ 20°) to the normal to the first surface), then a good approximation to the total angle, $D,$ of deviation is $$D=(n-1)A$$ in which $A$ is the prism angle (the angle between the faces through which the ray enters and leaves). $A$ must be small for the approximation to hold. $n$ is the refractive index of the material of the prism.
Philip Wood
- 36.6k
- 3
- 35
- 85