Optics deal with light in a classical way
(i.e, without quantum concepts) using one of two viewpoints:
- Geometrical optics
is based on the concept of light rays
propagating in a straight line according to the classical laws of reflection
(angle of reflexion = angle of incidence) and refraction
(Snell's law).
Both of these where unified as consequences of the principle of least time
postulated by Pierre de Fermat iaround 1635 and confirmed experimentally in 1851
(when it was finally established that the celerity of light is indeed inversely proportional
to the index of refraction of the medium).
-
The wave theory of light, on the other hand,
explains diffraction
(as well as the laws of reflexion and refraction of geometrical optics, incidentally).
It was first championned by Christiaan Huygens and
received experimental support from Thomas Young in 1803.
The idea that light is a form of electromagnetic wave is due to
Michael Faraday,
who was later vindicated mathematically by James Clerk Maxwell
(Maxwell's equations, 1864).
By contrast, photonics
is based on the explicit idea that light consists of
packets of energy proportional to its frequency
(the coefficient of proportionality being
Planck's constant).
This idea was formally put forth in 1905 by
Albert Einstein to explain the
photoelectric effect
(in 1900, Max Planck
had paved the way by showing that the blackbody spectrum
could be explained by postulating that
all energy exchanges between radiation and matter
could only occur in quanta of energy proportional to the frequency).
So, the key difference between optics and photonics is that the latter deals primarily
with the
quantization of light which is ignored by the former.
Also, in optics we consider light to consist either
of particles (explaining the
light rays and sharp shadows
on which geometrical optics is based)
or waves (which explain diffraction using Huygens principle).
In photonics, we integrate the quantum notion that the light quanta
(photons) have properties characteristic of both waves and particles.
Wikipedia :
Photonics
vs. Geometrical optics.
