What is Double-slit experiment ?

This phenomenon occurs even if the particles are fired one at a time, showing that the particles demonstrate some wave behavior by interfering with themselves as if they were a wave passing through both slits.

However, when this "single-slit experiment" is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the angle of spread.

The original double-slit experiment was performed using light/photons around the turn of the nineteenth century by Thomas Young, so the original experiment is often called Young's double-slit experiment

In the early nineteenth century, Thomas Young devised the double-slit experiment to prove that it consisted of waves. While the implications of Young's experiment are difficult to accept, they are illustrative of quantum theory.

The top portion of the image shows the central portion of the pattern formed when a red laser illuminates a slit and, if one looks carefully, two faint side bands. More bands can be seen with a more highly refined apparatus. Diffraction explains the pattern as being the result of the interference of light waves from the slit.

The idea of using particles other than photons in the experiment did not come until after the ideas of de Broglie and the advent of quantum mechanics, when it was proposed that fundamental particles might also behave as waves with characteristic wavelengths depending on their momenta.

The noted physicist Richard Feynman claimed that the essentials of quantum mechanics could be grasped from an exploration of the results of the experiment.

In the more modern version of the double slit experiment using electrons, electrons with the same momentum are shot from an "electron gun" like the ones inside CRT televisions towards a screen with two slits in it.

If one illuminates two parallel slits, the light from the two slits again interferes. Here the interference is a more pronounced pattern with a series of alternating light and dark bands. The width of the bands is a property of the frequency of the illuminating light.

In the double-slit experiment, a beam of light is aimed at a barrier with two vertical slits. After the light passes through the slits, the resulting pattern is recorded on a photographic plate. When one slit is covered, a single line of light is displayed, aligned with whichever slit is open.

. After each electron goes through one of the slits, it is observed hitting a single point on a detecting screen at an apparently random location. As more and more electrons pass through, one at a time, they form an overall pattern of light and dark interference bands.

Intuitively, one might hypothesize that if both slits were open, the resulting pattern would display as two lines of light, aligned with the slits. What occurs in practice, however, is that light passing through the slits and displayed on the photographic plate is entirely separated into multiple lines of lightness and darkness in varying degrees.

(See the bottom photograph to the right.) When Thomas Young (1773–1829) first demonstrated this phenomenon, it indicated that light consists of waves, as the distribution of brightness can be explained by the alternately additive and subtractive interference of wavefronts.

The result illustrates that interference is taking place between the waves and particles going through the slits in what a layman might expect to be two non-crossing trajectories.

If each electron was truly just a point particle, then there would only be two clusters of observations: one for the electrons passing through the left slit, and one for the right. However, if electrons are made of waves, they interfere with themselves and pass through both slits simultaneously. Indeed, this is what is observed when the double-slit experiment is performed using electrons.

Young's experiment, performed in the early 1800s, played a vital part in the acceptance of the wave theory of light, vanquishing the corpuscular theory of light proposed by Isaac Newton, which had been the accepted model of light propagation in the 17th and 18th centuries. However, the later discovery of the photoelectric effect demonstrated that under different circumstances, light can behave as if it is composed of discrete particles.

In Young's double-slit experiment, photons corresponding to light of wavelength are fired at a barrier with two thin slits separated by a distance as shown in the diagram below.

After passing through the slits, they hit a screen at a distance of away with and the point of impact is measured. Remarkably, both the experiment and theory of quantum mechanics predict that the number of photons measured at each point along the screen follows a complicated series of peaks and troughs called an interference pattern as below.

This is the conundrum quantum physicists have been puzzling over for many centuries, ever since photon-wave mechanics was theorized and the Double Slit experiment was first conducted.

Also known as Young’s experiment, this involved particle beams or coherent waves passing through two closely-spaced slits, the purpose of which was to measure the resulting impacts on a screen behind them. In quantum mechanics the double-slit experiment demonstrated the inseparability of the wave and particle natures of light and other quantum particles.

These seemingly contradictory discoveries made it necessary to go beyond classical physics and take the quantum nature of light into account.Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment. He also proposed (as a thought experiment) that if detectors were placed before each slit, the interference pattern would disappear

The photons must exhibit the wave behavior of a relative phase shift somehow to be responsible for this phenomenon. Below, the condition for which maxima of the interference pattern occur on the screen is derived.

The Double Slit experiment not only gave rise to the particle-wave theory of photons, it also made scientists aware of the incredible, confounding world of quantum mechanics

A low-intensity double-slit experiment was first performed by G. I. Taylor in 1909, by reducing the level of incident light until photon emission/absorption events were mostly non overlapping. A double-slit experiment was not performed with anything other than light until 1961, when Claus Jönsson of the University of Tübingen performed it with electron beams.

Any student of physics, and in particular quantum mechanics will tell you that reading articles in the media that concerns any element of quantum physics can be a minefield.