Wave particle duality

In the 17th century, scientists had a disagreement on whether light is particle or wave. Isaac Newton stated that light is the particle. He argued that since the reflection of the light is a perfectly straight line, it must have particle nature. The other group of scientists like Christiaan Huygens, however, argued that light has wave nature. Their argument based on the fact that light travels at a different speed in different media. This property of light demonstrates its wave nature. The view that light is wave was later supported by the discovery made by Thomas Young. In the double-slit experiment, Young discovered that light creates interference patterns when it passes through two narrow slits. This discovery confirmed the wave nature of the light. Interestingly, Einstein confirmed the particle nature of the light by introducing the "photoelectric effect" in 1905. In the photoelectric effect, it was observed that shining light on a piece of metal creates an electric current in the circuit. To explain the photoelectric effect, scientists assume that there are "light particles" knocking electrons out of the metal. An interesting thing about the photoelectric effect is that the energy of the electrons does not depend on the intensity of light. In addition to that, scientists found out that only the light above the threshold frequency could knock electron free. If we combine these two facts, we can think that one photon (light particle) of the light above threshold frequency could release only one photon. As we can see, photoelectric effect demonstrates the particle nature of light. At this point you may wonder: Who is right? Is light a wave or a bunch of particles? The answer is a little bit ambiguous. Both sides are not quite right but not entirely wrong either. Based on the fact that light has both wave-like and particle-like property, scientists invented a term, "wave-particle duality", to describe the property of light. In modern time, the explanation of wave-particle duality is widest accepted.

After learning that light has wave-particle duality. De Broglie took a bold step and claimed that all mater have both wave and particle-like property. He related the momentum and the wavelength, writing down a formula as follow: λ=h/p   h(Planck's constant), p(momentum), λ(wave length)

De Broglie's claim was proven three years later after the electron diffraction was observed. The fact that a beam of electrons passing through a thin film creates interference patterns proves that even electrons, something that is perceived as particles, has wave-like properties. Later experiments demonstrated that particles that are much heavier than electrons also create interference patterns after passing through a thin film. From the formula above, we can see that the larger the momentum of the object, the shorter the wavelength. Small particles like electrons have small momentum and long wavelength, so their wave property is more obvious. Large objects like baseball and human bodies have large momentum and short wavelength, so the wave property is not so obvious. 

Wave-particle duality is a bizarre phenomenon in the real world. It is also the gate to quantum mechanics. Through years of study, scientists have been revealing the mysterious nature of quantum physics little by little. The study of quantum physics helps us understand how the real world works and how particles behave at the microscopic level. The research on quantum mechanics can not only increase our understanding of this world but also advance the technology. Technology like quantum computer and teleportation may be achieved by applying quantum mechanics. Let's all stay tuned. Because a ground breaking discovery may appear at any time.