# What is the wave theory of light

## History of light

(Course level > light)

### What is light

People have been concerned with this question since the beginning of time. In ancient times the belief was widespread that rays of light emanate from the eyes and scan the surroundings during the process of seeing, or that light moves at infinite speed. But how exactly does light move? To answer this question, physics had to go a long way from particle theory and wave theory to quantum mechanics.

### Particle theory

The particle theory was particularly coined by Isaac Newton at the end of the 17th century and says that light consists of tiny particles, the "corpuscles". These are ejected in a straight line from luminous bodies, whereby the speed of light depends on the speed of the light source.

"Under Rays of light I understand the smallest particles of light, both one after the other in the same lines and simultaneously in different ones. For it is clear that light consists of both succesive and simultaneous particles, since the light arriving at a certain moment can be caught at the same point and at the same time the following can pass by, and in the same way one can pass the light on at the same moment Catch the spot and let it pass another. Because the light caught cannot be the same as the light that has passed. The smallest light or light particle, which can be caught for itself separately from the rest of the light, or alone does or suffers something that the rest of the light does not do, nor suffers - this I call a ray of light. "[1]

This theory can be used to explain various properties of light, such as the color of light, which is due to the different sizes of the particles. But this theory also gave a plausible explanation for the reflection of light. However, particle theory found its limits at diffraction and refraction. The attempt to explain refraction is that the light particles are attracted to different degrees when they pass from one medium with a certain density to another medium, thereby changing the direction of flight. This led to the assumption that light flies faster in the denser medium. Despite some ambiguities, particle theory was the favored one for a long time.

### Wave theory

The wave theory began with the discovery made in 1650 by Christiaan Huygens that a wave-like propagation of light would explain phenomena such as refraction, diffraction and interference plausibly. His formulated "Huygens principle" states that spherical waves (elementary waves) emanate from every point of a diffractive surface. If the wave font enters another medium, the speed of propagation changes and thus the direction of propagation. The wave is broken.

Breaking a wave

If the wave hits an obstacle, this leads to diffraction at the edge of the obstacle. This can be clearly seen in the example of a gap.

Diffraction of a wave

It was also more logical to Christiaan Huygens that light must be a wave, since light travels so incredibly quickly. But his theory was only laughed at in public, because Newton's influence with his particle theory was simply too great. Only when Thomas Young presented the results of his new experiment, the double slit experiment, in 1802, did the wave theory gain greater acceptance in society. Because with the proof that interference is a property of light that cannot be explained with the particle theory, the wave theory appeared more plausible. The ultimate breakthrough in wave theory came in 1873 when James Clerk Maxwell called light an electromagnetic wave. The so-called ether, which is supposed to be present everywhere, was used as the carrier medium. However, the Michelsen-Morley experiment (1881) refuted the existence of the ether. Some time later, Heinrich Hertz confirmed the electromagnetic theory of light with the help of various experiments.

### The theories - attempts

In order to check the correctness of the theories, various experiments must be carried out and these must be brought into line with one of the theories. The double slit test and the opposing field method are particularly suitable for this. The proven interference of light with the help of the double slit test proves the wave theory, since light consisting of particles would not form any interference phenomena. However, it looks completely different with the opposing field method!

Opposite field method
construction
Structure of the opposing field method

The light from a mercury vapor lamp shines through an interference filter onto a photocathode. This filters a narrow wavelength range that hits the cathode. A voltage is applied between the collecting anode and the photocathode via a voltage source.

If you switch on the lamp, electrons are knocked out of the cathode, which move to the anode and enter it due to their kinetic energy. Now you apply a counter voltage [math] U_0 [/ math] so that the electrons not only have to perform the work function [math] W [/ math] from the cathode, but also have to overcome the generated electric field. The counter voltage is increased until no more electrons arrive at the anode. With the determined voltage, the maximum kinetic energy of the electrons after exiting the cathode can now be calculated: [math] Epot = e \ cdot U_0 [/ math] This can now be done for different frequencies, the result is the following diagram, and thus the confirmation that the amount of energy transmitted by a light particle only depends on the frequency and not on the intensity ("amplitude"). This means that not every frequency range of light can produce the photo effect.

The following observations can be made:

• The kinetic energy of the electrons does not depend on the intensity of the light, but on its wavelength or frequency
• The kinetic energy of the photoelectrons increases linearly with the frequency of the light
• The electrons are knocked out of the cathode only a few nanoseconds after switching on the light

[math] \ Rightarrow [/ math] Light has a speed of propagation

The particle properties of light can be demonstrated on the basis of this experiment. But what is the light really? A wave or a particle? Light can be both, wave and particle, depending on the experimental setup. This inexplicable phenomenon is known as wave-particle dualism. Since light could not be explained with classical physics, the physicists had to develop other theories.

### 20th century light theory

Since the correctness of the particle theory and the wave theory was questioned at the end of the 19th century, other approaches had to be found. A first step in a promising direction was the quantification of energy (= energy “packages”) proposed by Max Planck around 1900. However, this only related to the energy contained in matter and not to light. Five years later Albert Einstein was researching the photo effect, which consists in the fact that light can release electrons from metal. The amount of energy that a single electron can get from the light beam is always the same and proportional to the frequency.

[math] \ Rightarrow [/ math] Light consists of "light quanta"

According to quantum theory, light is neither a wave nor a particle; it can take on the properties of both forms.