The wave particle duality, Slides of Physics

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The wave-particle duality

Newton VS Huygens

Corpuscular theory Wave theory

The different colors of light are caused by differentsizes and masses of the particles

The proponents of the wave theory had stated earlier that the light waves are composed of white light. The spectrum, which can be observed through a prism, is formed due to damage of the inside of the glass. This means that most of the light travels through the glass the large majority of it will be corrupted. In order to demonstrate that this was false, Newton made to pass a beam of white light through two prisms situated at an angle which remained divided in its spectrum passing through the prism, while recomposed again as white light, passing through the second.

This experiment showed that the spectrum is not caused by the corruption of light when

it passes through a glass. Newton claimed to be a "crucial experiment". A crucial

experiment is any experiment designed to decide between two contradictory theories, for

which the failure of the first determines the success of the second. As everybody agreed

on the fact that the light should be made by particles or waves, Newton used the failure

of the wave theory to show that the light is made of particles.

He concluded that the light is composed of colored particles, which combine appearing

white. He introduced the term of the color spectrum and ,although the spectrum appears

continuous with no distinct boundaries between colors, he decided to break it up into

seven colors: red, orange, yellow, green, blue, indigo and violet. Newton choose the

number seven because this one is considered a mystical numbers by ancient greek and it

is also correlated with seven "wandering stars", seven days of the week, and a quarter of

the time between two full moons.

Newton demonstrated that each color has a unique angle of refraction, which can be calculated using a suitable prism. He observed that all objects seemed to be the same color of the beam of colored light that illuminated them, and this one will remain to the same color, no matter how many times it has been reflected or refracted. This led him to conclude that the color is a property of light, reflected by the objects, and not a property of the objects themselves. Although Newton was convinced of the validity of his theory, this was not immediately accepted and several were the problems that had to face and solve.

Wave theory (1678)

The Dutch mathematician Christiaan Huygens claimed to have disproved Newton's theory, showing that the laws of reflection and refraction could be derived from his wave theory of light. Huygens argued that the diffraction occurs is caused by the interference of the wave-fronts. When light passes through a small hole, the waves are held together from different angles and this generates fringes of light and dark shadows.

The main features of Huygens’s wave theory:

Every point of the given wave-front called 'primary wave- front' acts as a fresh source of new disturbance, called 'secondary wavelets‘, which travel in all directions with the velocity of light in the medium.

A surface touching these secondary wavelets tangentially in the forward direction at any instant gives a new wave-front at that instant. This is the secondary wave front.

Huygens was able to explain refraction and double refraction in the crystal "spar of Iceland ": placing, for example, a crystal of Iceland spar (calcite transparent and colorless) on one page printed, you can observed that all words are split. This means that the light rays from the page that are not forwarded by the uniquely crystal block: entering in Iceland spar of each beam it is divided in two and therefore the crystal provides a double image of each object behind it.

Studying more thoroughly the phenomenon, it is founded that the two into one follows the normal laws of ray refraction, and is therefore said ordinary ray, while the other does not follow at all is said and extraordinary ray.

Young was able to check with the necessary experimental precision

that the light filtered through two small holes very close to each

other, was able to produce a completely analogous interference

phenomenon to the light produced by two circular wave sources. This

experiment led to the evidence of the wave nature of light.

Double slit experiment

The light forms an image on a screen that has a maximum central bright and a series of side fringes alternately dark and light.

The interference can be explained only through the wave theory.

Einstein: photons

In 1905, Albert Einstein interpreted the experimental results of the photoelectric effect using a completely new conceptual model of electromagnetic radiation, based on the hypothesis put forward by Max Planck for the issue of the black body.

Plank had supposed that the interaction between matter and radiation happen through discrete packets, or quanta of energy.

Einstein founded that the very nature of the energy transported by the electromagnetic radiation was not a continuous stream, but a stream of discrete quanta or photons, that is, energy packages proportional to the frequency of the radiation.

A beam of light of frequency f consists of photons whose energy is given by the following relation:

E = h·f = h·(c/λ)

costante di Plank

frequenza velocità della luce

lunghezza d’onda

Experiments show that:

 to eject the electrons of the light beam you must have a frequency

greater than a specified minimum value, the threshold

frequency f

 if the frequency is the greater number of the release, electrons is

greater too

E = h·f

Threshold frequency f

f