 # Wave theory of light

## Introduction

The wave theory of light was a way scientists understood light. The theory was first spread by Christiaan Huygens and Robert Hooke in the 17th century. They at that time predicted that the light was a wave as it could refract or bend when travelling from one medium to another, reflect off shiny surfaces, diffract around objects, etc. The observers can also see interference where troughs or peaks of waves add up to produce brighter light or cancel out to create darker regions. However, in the late 17th century, many scientists were confused about the fundamental nature of light. They were entangled in a debate whether the light was a wave or a particle.

In that instance, Sir Issac Newton was in favour of the particle nature of light. But, Christiaan Huygens, the Dutch physicist, believed that the light is made up of waves vibrating up and down in direction perpendicular to the direction of the wave propagation. Based on that belief, he formulated a way of visualizing wave propagation, which then came up as the Huygens’ Principle. The wave theory of light given by Huygens has withstood the tests of time, and today, considered as the backbone of optics. Here, in this article, let us have a detailed insight into the wave theory of light.

Light always stimulated the curiosity of not just scientists but also thinkers and viewers. But, it was in the late 17th century that scientists and specialists began to understand the properties of light. On the one side, Christian Huygens believed that the light was made up of waves propagating in the direction perpendicular to the direction of its movement, Sir Issac Newton on the other side proposed that it consists of tiny particles known as the photons.

In 1678, Huygens suggested that each point a luminous disturbance meets would itself turn into a source of the spherical wave. The sum of the secondary waves resulting due to the disturbance would determine what form the new wave will take. This theory of light is known as Huygens’ Principle.

Using his principle, Huygens succeeded in deriving the laws of refraction and reflection of light. He was also successful in using his theory to explain both the linear and spherical propagation of light. Nevertheless, he was unable to describe the diffraction effects of light. After that, the experiment conducted in 1803 by Thomas Young on the interference of light proved that Huygens' wave theory of light was correct. Moreover, Fresnel in 1815 provided mathematical equations for Young’s experiment.

Later, Max Planck came up with another theory and proposed that light is made of finite packages of energy known as a light quantum, which depends on the velocity and frequency of light. Following others, Einstein, in 1905, proposed light as something that possessed the characteristics of both wave and particle. Quantum mechanics later gave proof of the dual nature of light. The major weakness of the wave theory of light was that light waves, like sound waves, need a medium for transmission.  The existence of the hypothetical compound luminiferous Aether proposed in 1678 by Huygens was cast into doubt in the late 19th century by the Michelson – Morley experiment. Newton's corpuscular theory proposed that light travel faster in a denser medium, whereas the wave theory given by Huygens and others implied the opposite. As the speed of light at that time could not be measured accurately, it was difficult to decide which theory would be correct. Léon Foucault was the first one to make an accurate measurement in 1850. The result provided by him supported the wave theory, and the particle theory was finally abandoned, just to partially re-emerge in the 20th century.

Light Wave Theory

Since light behaves as a wave and is made up of both electric and magnetic fields, it is categorized as the electromagnetic wave in most of the cases. Electromagnetic fields oscillate perpendicularly to the direction in which the wave travel and are also perpendicular to each other. Hence, they are known as transverse waves. Following are some significant characteristics of light :

• Light has a definite speed, i.e., the speed of light can never change on its own.

• A single beam of light can travel the earth around 7.5 times in 1 second.

• Like almost all other electromagnetic waves, light waves also travel with a speed of 3.0 x 108 m/s.

• A light-year is the distance that light waves travel in one year.

• To deal with light waves, we need to consider a sine waveform.

• Brightness or we can say the intensity of light is represented by amplitude and depends on the distance and how much light the source produces.

• The light emitted from a source is measured lumens.

• The wavelength of light waves is shorter than infrared waves.

• As per the formula devised by Planck, the energy of a photon is directly proportional to the frequency of light and is given as:

E= hf, where h is the Planck’s constant 6.63×10⁻³⁴ Joule - Second.