Laws of Reflection, Refraction, and Light's Dual Nature

Laws of reflection and refraction
When a wave strikes the surface between two media of different refractive index, part of the wave is reflected and partly refracted (transmitted by other means). The laws of reflection and refraction tells us that:
· · Rays incident, refracted and reflected are on the same plane, called plane of incidence, which is perpendicular to the surface.
· • The angle of incidence, Oi, and the angle of reflection, Orson equal.
· • The angle of incidence and transmission / refraction angle, Ot are related by Snell's law: n1senOi = n2senOt, where n ₁ and n ₂ are the indexes on the first and second means.

Snell's law implies that if light passes half of higher index, the rays are close to normal (away from the norm if the second medium has a lower rate).
Snell's law can also be expressed in terms of the velocities of light in the two media, given that n = clv

Photon concept. Wave-particle duality
To explain certain phenomena of emission and absorption of light by matter, including the photoelectric effect, Einstein returned to the particle theory of the nature of light. He assumed that the energy of electromagnetic radiation was not continuous but discrete, so that an electromagnetic wave of frequency v, could be considered consists of quanta or corpuscles that travel at the speed of light, each of which has an energyE = hv (where h is Planck's constant) and a momentum p = h / A. For these quanta are called photons.
Einstein's theory does not invalidate the electromagnetic theory of light. Modern physics has had to introduce the wave-particle duality, admitting that light has both wave and particle qualities. When light interacts with matter behaves like a stream of particles (photons) with energy and momentum, when spreads or suffers diffraction or interference, light behaves like a wave characterized by its wavelength and frequency.
Later, de Broglie proposed for reasons of symmetry that matter also has wave-particle duality, so that any particle has an associated wave. The associated wavelength is very small at macroscopic scales, so that the wave character of matter appears only at the microscopic level.