The branch of physics which deals with the study of optical phenomenon like reflection, refraction, dispersion, interference, diffraction, polarization etc. is known as optics. Optics is classified into two categories: the geometrical optics and physical optics.
Geometrical optics is a branch of optics which deals with the study of optical phenomenon without considering the nature of light.
For example, reflection and refraction of light are discussed under geometrical optics.
Physical optics is a branch of optics which deals with the study of optical phenomenon by taking into account the nature of light.
For example, interference, diffraction and polarization of light are discussed under physical optics.
Optical Path and Geometrical Path
The actual distance traveled by light in a given medium is known as geometrical path.
Optical path of light in a given medium is defined as the product of the geometrical path and refractive index of the given medium.
Consider a medium of thickness ‘t’ and refractive index n. Let, light pass through the medium from point A to point B. The geometrical path of light in the given medium = AB = t. The optical path of light in the given medium,
Since n>1, therefore, in optical path and geometrical path ‘optical path > geometrical path’