Light has always aroused the interest and curiosity of human beings. Thus, since its creation, one of the fundamental problems of physics has been to reveal the mysteries of light.
However, it was not until the end of the 17th century and the beginning of the 18th century, with the theories of Isaac Newton and Christiaan Huygens, that they began to lay the foundations for a deeper knowledge of light.
Principles of Huygens Wave Light Theory
In 1678, Christiaan Huygens formulated his theory of the waves of light, which later, in 1690, he would publish in his work Treatise on Light.
The Dutch physicist proposed that light was emitted in all directions as a set of waves moving through a medium he called the ether. As waves are not affected by gravity, it was assumed that the speed of the waves was reduced when they entered a denser medium.
His model was particularly useful in explaining the Snell-Descartes law of reflection and refraction. It also satisfactorily explained the phenomenon of diffraction.
His theory was fundamentally based on two concepts:
a) Light sources emit spherical waves, similar to the waves that occur on the surface of water. Thus, light rays are defined by lines whose direction is perpendicular to the wave surface.
b) Each point on a wave is, in turn, a new emitting center of secondary waves, emitted with the same frequency and speed that characterized the primary waves. The infinity of the secondary waves is not perceived; therefore, the resulting wave of these secondary waves is their envelope.
However, the theory of Huygens waves was not accepted by scientists of his time, with few exceptions, such as Robert Hooke’s.
Newton’s enormous prestige and the great success that his mechanics achieved, together with the problems in understanding the concept of ether, made most contemporary scientists opt for the corpuscular theory of the English physicist.
Reflection is an optical phenomenon that occurs when a wave has an oblique impact on a separating surface between two media and undergoes a change of direction, returning to the first medium together with part of the energy of the movement.
The reflected ray, the incident, and the normal (or perpendicular) are located on the same plane.
The value of the angle of incidence is exactly the same as the angle of reflection.
Huygens’ principle allows us to demonstrate the laws of reflection. It is verified that when a wave reaches the separation of the media, each point becomes a new emitter focus, emitting secondary waves. The reflected wavefront is the envelope of the secondary waves. The angle of this reflected secondary wavefront is exactly the same as the angle of incidence.
However, refraction is the phenomenon that occurs when a wave has an oblique impact on a space between two media, which have a different refractive index.
When this happens, the wave penetrates and is transmitted through the second medium, along with some of the energy of the movement. Refraction occurs as a result of the different speed with which waves propagate on different media.
A typical example of the refraction phenomenon can be observed when an object (eg a pen or pen) is partially introduced into a glass of water.
Huygens’ principle provided a convincing explanation of refraction. Points on the wavefront located on the boundary between the two media act as new sources of light propagation and therefore the direction of propagation changes.
Diffraction is a physical phenomenon characteristic of waves (occurs in all types of waves) which consists in the deflection of waves when they encounter an obstacle in their path or pass through a crack.
It should be taken into account that diffraction only occurs when the wave is distorted due to an obstacle whose dimensions are comparable to its wavelength.
Huygens’ theory explains that when light hits a slit, all points on its plane become secondary sources of waves, emitting, as explained above, new waves which, in this case, are called diffracted waves.
The Unanswered Questions of Huygens Theory
Huygens’ principle left a series of questions unanswered. His claim that each point on a wavefront was, in turn, a source of a new wave, did not explain why light travels both backwards and forwards.
Likewise, the explanation of the ether concept was not entirely satisfactory and was one of the reasons why his theory was not initially accepted.
Wave model recovery
It wasn’t until the 19th century when the wave model was recovered. It was mainly thanks to the contributions of Thomas Young, who managed to explain all the phenomena of light based on the fact that light is a longitudinal wave.
Specifically, in 1801, he performed his famous double-slit experiment. With this experiment, Young found an interference pattern in light from a distant light source when diffracted after passing through two slits.
In the same way, Young also explained, through the wave model, the scattering of white light in the different colors of the rainbow. He showed that in each medium, each of the colors that make up light has a characteristic frequency and wavelength.
In this way, thanks to this experiment, he demonstrated the nature of light waves.
Interestingly, over time, this experiment proved essential to demonstrate the dual-wave light corpuscle, a fundamental feature of quantum mechanics.