Theory of Light



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 ), X-Rays, 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

Electromagnetic Wave Propagation Diagram.  Flavio Spedalieri

Waves have two important characteristics - wavelength and frequency.


The Sine Wave

Sine Wave Diagram.  Flavio Spedalieri

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:

l = c / = c / l

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=MC2 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 Joule-Second.
 

E = h

or

E = hc / l


E is the photonic energy in Joules, h is Planks constant  and f is the frequency in Hz