Snell’s law (or the law of refraction) is one of the fundamental laws of geometric optics. It states that when a ray of light hits an interface dividing two transparent media and refracts at the interface, the transmitted ray remains within the plain of incidence. Additionally, the sines of the angles wrt the surface normal are proportional to each other:
where is the incident angle, and is the transmitted angle.
One note about computations. Typically, objects emit several rays towards an interface (which may then converge towards a point or diverge). This means finding the angle in problems is usually unworkable, because there are many angles at play in the question.