Wave-Particle Duality

We mentioned in our previous article that Newton’s laws of motion constitute the foundations of classical physics. All scientist accepted these laws. It was the first mathematical expression of nature. These laws were very successful on visible objects (balls, planets…). In the 17th century, scientists attempted to describe the atoms that make up invisible matter. Scientists who tried to apply Newton’s laws of motion to subatomic particles were disappointed. Likewise, Newton’s laws of motion were insufficient at this point. And the discussions began. On what basis was the behavior of subatomic particles based?

Issac Newton
Issac Newton (1642-1727)

Discovery of subatomic particles

While discussions were held on the behavior of matter, there was no precise information about the particles that make up matter. The discovery of the atom and its modeling have shown a historical development. The correct understanding of the structure of the atom has also shed light on the understanding of wave-particle duality. First, let’s talk briefly about the historical development of the structure of the atom.

the smallest particle of matter: atom
Atom diagram

Historical development of the structure of the atom:

  • Electron was first discovered in 1987 by J.J Thomson’s cathode experiment. He applied a high potential difference by placing two metals in an evacuated glass tube. J.J Thomson was studying the emitted rays at this time. Thomson saw that the particles that make up these rays are negative and named them ‘electrons’.
  • Thomson discovered the electron, but fell short of where the electrons are located in the atom. According to Thomson, electrons were above the atom. Thomson likened it to raisin cake. In 1908, Ernest Rutherford disproved the raisin cake model with his alpha scattering experiment. The positively charged alpha particles he sent to the metal surface showed different movements. Ninety percent of the particles sent deviated very slightly, while some bounced back in the same direction. As a result of this experiment, the nuclear model of the atom emerged. Accordingly, the atom has a positively charged nucleus and is surrounded by electrons.
Rutherford's atom experiement

Diagram by Steven B. Krivit showing reaction process as observed and interpreted by Ernest Rutherford in 1919.
  • The uncertainty principle has changed the direction of the understanding of the atomic model. In this direction, Schrödinger created a different model of the atom. According to Schrödinger, electrons are on an energy cloud (orbital) that has kinetic energy, revolving around itself. Electrons can move to a different energy level and emit rays at the same time. Thus, the modern atomic model formed.

What is wave-particle duality?

Wave-particle duality is about the behavior of light. In the 19th century, there was a lot of discussion about whether light is a particle or a wave. The photoelectric effect was strong evidence that light is a particle.
Photoelectric effect: Electron ejection from metal particles when ultraviolet rays are sent to a metal surface. The ejected electrons contain the energy of the transmitted beam and these electrons are called photoelectrons. This observation of Einstein brought the understanding that electromagnetic radiation consists of photons.


Photons: They are packets of energy that make up electromagnetic radiation. Proves that radiation exhibits particle behavior.

Electromagnetic waves
Electromagnetic waves

Electromagnetic Radiation: Electromagnetic radiation is self-propagating waves in a vacuum or matter. It occurs as a result of the accelerated motion of a charged particle. There are electric and magnetic field components perpendicular to each other and spread in the perpendicular direction to the plane formed by these two fields. They do not require a medium for their propagation, they are transverse waves propagating at the speed of light c in vacuum.

Young’s Two-Slit Experiment

Young’s Double-slit experiment

Thomas Young proved the wave behavior of light with his double slit experiment. If we want to explain the experiment in a simple way; A light source, such as a laser beam, is reflected on a thin plate with two parallel slits, and light passing through the slit is reflected on a screen behind the plate. As a result, light and dark bands appear on the screen. This proves the wave behavior of light. That is, when the crests of the two waves coincide, they grow and increase in intensity. It shows the bright part on the screen.

When the crests and troughs of the waves overlap, they extinguish each other. It shows the dark part on the screen. If light exhibited particle behavior, there would be a single line on the screen. However, the beam particles sent form an interference pattern on the screen, just like a water wave. This proves that the beam particles exhibit both wave and particle motion.

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