Ray optics or geometrical optics

Geometrical (ray) optics is mainly concerned with image formation by mirrors, lenses and prisms, and the whole subject rests on four fundamental postulates, already met in earlier classes.

1. Rectilinear propagation: light travels in a straight line in a homogeneous (properties the same everywhere) and isotropic (properties the same in every direction) medium.
2. Independence of rays: two or more light rays can cross at a point without affecting each other's paths beyond that point of intersection.
3. Laws of reflection: (a) the reflected ray lies in the plane formed by the incident ray and the normal drawn at the point of incidence, with the incident and reflected rays on either side of the normal; (b) the angle of incidence equals the angle of reflection.
4. Laws of refraction (these apply at the boundary between any two media): (a) the refracted ray lies in the plane formed by the incident ray and the normal drawn at the point of incidence, with the incident and refracted rays on either side of the normal; (b) Snell's law relates the angle of incidence $\theta_1$ in a medium of refractive index $n_1$ to the angle of refraction $\theta_2$ in a medium of refractive index $n_2$: $n_1\sin\theta_1 = n_2\sin\theta_2$. Remarkably, all four of these laws can be derived from one single, deeper principle -- Fermat's principle -- which states that while travelling from one point to another by one or more reflections or refractions, a ray of light always follows the path of least time (more precisely, of EXTREME, i.e. minimum or maximum, time). …