The wave theory of light, proposed by Dutch physicist Christian Huygens in 1678, has stood the test of time and is today considered one of the backbones of optics.
Definition of Light
Before going into the wave theory of light, let’s learn about light itself. You may be aware by now that light is a form of energy. It is the only form of energy that is visible to the naked eye. In more precise terms, light is a part of the electromagnetic spectrum, which includes visible light, radio waves (AM, FM and SW), microwaves, x-rays and gamma rays.
Background to the Wave Theory of Light
Light has always intrigued scientists and thinkers. But it was not until the late 17th century that scientists began to study its properties. Sir Isaac Newton thought that light was made of tiny particles. But Dutch physicist Christian Huygens believed that it was made of waves that propagated by vibrating up and down perpendicular to the direction of its movement.
In 1678, Huygens proposed that every point that a luminous disturbance meets turns into a source of the spherical wave itself. The sum of the secondary waves, which are the result of the disturbance, determines what form the new wave will take. This theory of light became known as ‘Huygens’ Principle’.
Using this principle, Huygens was successful in deriving the laws of reflection and refraction of light. He was also successful in providing an explanation for the linear and spherical propagation of the light waves. However, he was not able to explain the diffraction effects. As you may already know, diffraction effects are the bending of light waves that take place when they hit the edges of an obstacle or opening, screens and apertures. The explanation was provided by French physicist Augustin-Jean Fresnel in 1816.
Huygens’ Principle was proved to be correct by Thomas Young in 1803. While studying the interference of light, he showed that when light is shone through a screen with two slits that are equally separated, the light emerging from them spread out according to Huygens’ Principle. In 1815, Fresnel provided mathematical equations for Young’s experiment.
In 1900, physicist Max Planck proposed that light was made of finite packets of energy, called a light quantum, which depends on the velocity and frequency of light. In 1905, Einstein proposed that light had the characteristics to support both particle and wave theory. Based on Planck’s work on the emission of light from hot bodies, he suggested that light is made of tiny particles called photons.
Later, quantum mechanics gave proof of the dual nature of light.
Huygens’ Principle
Huygens’ Principle (also called Huygens-Fresnel Principle) states that every point on a wavefront is a source of wavelets that spread forward at the same speed. It can be shown mathematically by the following equation:
s = v x t
where s is the distance travelled, v is the speed of propagation and t is the time.
Each point on a wavefront emits semi-circular waves at speed v. The waves which occur after some time, t. These waves create a new wavefront, which is tangent to the wavelets.
This principle works for all types of waves, including sound waves, and is helpful in describing the other phenomena of light, namely reflection, refraction and interference.
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