Double slit experiment has been conducted with different experimental setup
several times in history and it has been debated and interpreted in a bid to understand the quantum theory. The experiment has generated the principle of complementarity, wave-particle duality, quantum consciousness and parallel universes.
The two slit experiment has, for over a century, served to understand the workings of the microscopic world. In the early nineteenth century, Thomas Young set up a point source of light to illuminate two narrow adjacent slits and observe its image on a second screen.
The aim of the experiment was to study the nature of light: was it a wave as Huygens had suggested or was it made of particles as Newton had postulated?
If it was a wave, we would see interference fringes of dark and light bands, but if it was made of particles we would only see two bands corresponding to the two slits it would have passed through.
Sure enough, Young observed bands of light dark fringes formed by the superposition of two waves of light emerging from the two slits.
At the time, this seemed to validate Huygen’s wave theory of light, but subsequent attempts to detect ether, an invisible medium through which light was thought to propagate were in vain. Also, the classical theory of electromagnetism (wave theory) as formulated by Maxwell had assumed continuous absorption and emission of electromagnetic radiation, but the observed results were at variance with the theoretically expected results.
A century later Einstein’s explanation of the photoelectric effect (which occurs when matter emits electrons upon exposure to electromagnetic radiation, such as photons of light) turned the table once again in favor of the particle nature of light.
Einstein said the radiation falling on matter and emitted from it happened in discrete packets or quanta of energy called photons. The equation he developed with this assumption explained the test results. Later, when Niels Bohr developed his atomic model, he made use of Einstein’s equation and the quantization of energy emission.
Thus it was proved that light as well as atomic particles exhibit both wave and particle nature.
Wave or particle?
The basic double slit experiment outlined earlier was modified so that a series of single photons (light particles) are fired at a solid plate that has two slits. On the other side of the solid plate, a photographic plate is set up to record what comes through those slits.