What is light ?
Light is part of the electromagnetic
spectrum, the spectrum is the collection of all
waves, which include visible light, Microwaves, radio waves ( AM, FM, SW ), XRays,
and Gamma Rays.
In the late 1600s, important questions were raised, asking if light is made up of
particles, or is it waves .?
Sir Isaac Newton, held the theory that light was
made up of tiny particles. In 1678, Dutch physicist, Christiaan Huygens, believed
that light was made up of waves vibrating up and down perpendicular to the direction
of the light travels, and therefore formulated a way of visualising wave
propagation. This became known as 'Huygens' Principle'. Huygens theory was the successful theory of light
wave motion in three dimensions. Huygen,
suggested that light wave peaks form surfaces like the layers of an onion. In a
vacuum, or other uniform mediums, the light waves are spherical, and these wave
surfaces advance or spread out as they travel at the speed of light. This theory
explains why light shining through a pin hole or slit will spread out rather
than going in a straight line
(see diffraction). Newton's
theory came first, but the theory of Huygens, better described early experiments.
Huygens' principle lets you predict where a given wavefront will be in the
future, if you have the knowledge of where the given wavefront is in the
present.
At the time, some of the experiments conducted on light
theory, both the wave theory and particle theory, had some unexplained phenomenon, Newton
could not explain the phenomenon of light
interference,
this forced Newton's particle theory in favour of the wave theory. This difficulty was due
to the unexplained phenomenon of light Polarisation  scientists were familiar with
the fact that wave motion was parallel to the direction of wave travel, NOT perpendicular
to the to the direction of wave travel, as light does.
In 1803, Thomas Young studied the interference of
light waves by shining light through a screen with two slits equally separated, the light
emerging from the two slits, spread out according to
Huygen's principle. Eventually the
two wave fronts will overlap with each other, if a screen was placed at the point of the
overlapping waves, you would see the production of light and dark areas (see interference).
Later in 1815, Augustin
Fresnel supported Young's experiments with mathematical calculations.
In 1900 Max Planck proposed the existence of a light quantum, a finite packet of energy which
depends on the frequency and velocity of the radiation.
In 1905 Albert Einstein had proposed a
solution to the problem of observations made on the behaviour of
light having characteristics of both wave and particle theory.
From work of Plank on emission of light from hot bodies,
Einstein suggested that light is composed of tiny particles
called photons, and each photon has energy.
Light theory branches in to the physics of
quantum mechanics, which was conceptualised in the twentieth
century. Quantum mechanics deals with behaviour of nature on the
atomic scale or smaller.
As a result of quantum mechanics, this gave
the proof of the dual nature of light and therefore not a
contradiction.
Light Wave Theory
Light can exhibit both a wave theory, and a particle theory at the same time.
Much of the time, light behaves like a wave. Light waves are also called electromagnetic waves
because they are made up of both electric (E) and magnetic (H) fields. Electromagnetic fields
oscillate perpendicular to the direction of wave travel, and perpendicular to each other.
Light waves are known as transverse waves as they oscillate in the direction traverse to
the direction of wave travel.
The
Electromagnetic Wave
Waves have two important characteristics  wavelength and frequency.
The
Sine Wave
The sine wave is the
fundamental waveform in nature. When dealing with light
waves, we refer to the sine wave. The period (T) of
the waveform is one full 0 to 360 degree sweep. The
relationship of frequency and the period is given by the
equation:
f =
1 / T
T =
1 / f
The waveforms are always in the time domain
and go on for infinity.
Wavelength: 
This is the distance between peaks of a wave.
Wavelengths are measured in units of length  meters, When dealing with
light, wavelengths are in the order of nanometres (1 x 10^{9})

Frequency: 
This is the number of peaks that will travel past a
point in one second. Frequency is measured in cycles per second. The term given to this is
Hertz (Hz) named after the 19th century discoverer of radio waves  Heinrich Hertz.
1 Hz = 1 cycle per second 
The speed of a wave can be found by multiplying the two units together. The wave's speed is measured in units of length (distance) per second:
Wavelength x Frequency = Speed
The Speed Of Light
The speed of light in a vacuum is a universal constant, about 300,000 km/s or
186,000 miles per second. The exact speed of light is: 299,792.458 km/s
It takes approximately 8.3 min for light from the sun the reach the earth ( 150,000,000
/ 300,000 / 60 = 8.3 )
Taking the distance of the sun from Earth into account, which is 150,000,000 km, and
the fact that light travels at 300,000 km/s, it shows in someway how fast light actually
travels.
With the use of the SI units for wavelength (l), frequency (¦) and speed of light (c), we can derive
some simple equations relating to wavelength, frequency and speed of light:
Photon Model of Light
As proposed by Einstein, light is composed of photons, a very
small packets of energy. The reason that photons are able to travel at light speeds is due
to the fact that they have no mass and therefore, Einstein's
infamous equation  E=MC^{2 }cannot
be used. Another formula devised by
Planck, is used to describe the
relation between photon energy and frequency  Planck's Constant (h)
 6.63x10^{34} JouleSecond.
E
=
h¦
or
E
=
hc / l
E
is the photonic energy in Joules,
h
is Planks constant and
f
is the frequency in Hz
