Q.Which phenomena can be satisfactorily explained using ray optics? State the assumptions on which ray optics is based.

[!TLDR] Ray optics explains reflection, refraction, double refraction and TIR, resting on rectilinear propagation, independence of rays, and the laws of reflection/refraction (all following from Fermat's principle). [!ANSWER] Reflection, refraction, double refraction, total internal reflection; assumptions: rectilinear propagation in a homogeneous/isotropic medium, independence of intersecting rays, laws of reflection, laws of refraction (Snell's law).

Ray (geometrical) optics satisfactorily explains reflection, refraction, double refraction, and total internal reflection -- phenomena that can all be modelled using straight-line rays and simple angle relations, without needing to treat light as a wave at all. It rests on four assumptions: (i) rectilinear propagation -- light travels in a straight line through any homogeneous (uniform everywhere) and isotropic (uniform in every direction) medium;

(ii) independence of rays -- two or more rays crossing at a point do not affect each other's subsequent paths;

(iii) the laws of reflection -- the reflected ray is coplanar with the incident ray and normal, on the opposite side of the normal, with equal angles of incidence and reflection;

(iv) the laws of refraction -- the refracted ray is likewise coplanar with the incident ray and normal, on the opposite side, related by Snell's law n1 sin(theta1) = n2 sin(theta2). All four of these postulates can in fact be derived from Fermat's single, deeper principle of least (extreme) time. [!ANSWER] Reflection, refraction, double refraction, TIR; assumptions: rectilinear propagation, ray independence, laws of reflection, laws of refraction (Snell's law).

List the phenomena explainable by treating light purely as straight-line rays, then state the four postulates (rectilinear propagation, ray independence, reflection laws, refraction laws) that ray optics is built on.

Including wave-only phenomena (interference, diffraction, polarization) in the list of things ray optics explains, when these specifically require wave optics instead.