The double-slit experiment consists of letting light diffract through two slits producing fringes on a screen. These fringes or interference patterns have light and dark regions corresponding to where the light waves have constructively and destructively interfered. The experiment can also be performed with a beam of electrons or atoms, showing similar interference patterns; this is taken as evidence of the "wave-particle duality" explained by quantum physics.
Importance to physics
Although the double-slit experiment is now often referred to in the context of quantum mechanics, it was originally performed by the English scientist Thomas Young some time around 1805 in an attempt to resolve the question of whether light was composed of particles (the "corpuscular" theory ), or rather consisted of waves travelling through some aether, just as sound waves travel in air.
The interference patterns observed in the experiment seemed to discredit the corpuscular theory, and the wave theory of light remained well accepted until the early 20th century, when evidence began to accumulate which seemed instead to confirm the particle theory of light.
The double-slit experiment, and its variations, then became a classic Gedankenexperiment (thought experiment) for its clarity in expressing the central puzzles of quantum mechanics; although in this form the experiment was not actually performed until 1961 (using electrons), and not until 1974 in the form of "one electron at a time", in a laboratory at the University of Milan, by researchers led by Pier Giorgio Merli, of LAMEL-CNR Bologna.
The results of the 1974 experiment were published and even made into a short film, but did not receive wide attention. The experiment was repeated in 1989 by Tonomura et al at Hitachi in Japan. Their equipment was better, reflecting 15 years of advances in electronics and a dedicated development effort by the Hitachi team. Their methodology was more precise and elegant, but the results are unmistakably the same. Tonomura wrote that the Italian experiment had not detected electrons one at a time, a key to demonstrating the wave-particle paradox, but single electron detection is clearly visible in the photos and film taken by Merli and his group.
In September 2002, the double-slit experiment was voted "the most beautiful experiment" by readers of Physics World.
Explanation of experiment
In Young's original experiment, sunlight passes first through a single slit, and then through two thin vertical slits in otherwise solid barriers, and is then viewed on a rear screen.
When either slit is covered, a single peak is observed on the screen from the light passing through the other slit.
But when both slits are open, instead of the sum of these two singular peaks that would be expected if light were made of particles, a pattern of light and dark fringes is observed.
This pattern of fringes was best explained as the interference of the light waves as they recombined after passing through the slits, much as waves in water recombine to create peaks and swells. In the brighter spots, there is "constructive interference", where two "peaks" in the light wave coincide as they reach the screen. In the darker spots, "destructive interference" occurs where a peak and a trough occur together.
The thought experiment
By the 1920s, various other experiments (such as the photoelectric effect) had demonstrated that light interacts with matter only in discrete, "quantum"-sized packets called photons.
If sunlight is replaced with a light source that is capable of producing just one photon at a time, and the screen is sensitive enough to detect a single photon, Young's experiment can, in theory, be performed one photon at a time -- with identical results.
If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a screen. But when the experiment is arranged in this way, the fringes disappear -- for reasons related to the collapse of the wave function.
Conditions for interference
The two slits must be close to each other (about 1000 times the wavelength of the source), otherwise the spacing of the interference fringes would be too narrow to discern the interference pattern.
A necessary condition for obtaining an interference pattern in a double-slit experiment concerns the difference in pathlength between two paths that light can take to reach a zone of constructice interference on the viewing screen. This difference must be the wavelength of the light that is used, or a multple of this wavelength. If a beam of sunlight is let in, and that beam is allowed to fall immediately on the double slit, then the fact that the Sun is not a point source degrades the interference pattern. The light from a source that is not a point source behaves like the light of many point sources side by side. Each can create an interference pattern, but the interference patterns of each of the many-side-by-by-side sources does not coincide on the screen, so they average each other out, and no interference pattern is seen.
The presence of the first slit is necessary to ensure that the light reaching the double slit is light from a single point source. The path length from the single slit to the double slit is equally important for obtaining the interference pattern as the path from the double slit to the screen.
Newton's rings show that light does not have to be coherent in order to produce an interference pattern. Newton's rings can be readily obtained with plain sunlight. See the External links section More rings are discernable if for example light from a Sodium lamp is used, since Sodium lamp light is only a narrow band of the spectrum. Light from a Sodium lamp is incoherent. Other examples of interference patterns from incoherent light are the colours of soap bubbles and of oil films on water.
The width of the slits is usually slightly smaller than the wavelength (λ) of the light, allowing the slits to be treated as point-sources of spherical waves, and reducing the effects of single slit diffraction on the results.
In general, interference patterns are clearer when monochromatic or near-monochromatic light is used. Laserlight is as monochromatic as light can be made, therefore laserlight is used to obtain an interference pattern.
If the two slits are illuminated by coherent waves, but with polarizations perpendicular with respect to each other, the interference pattern disappears.
The bright bands observed on the screen happen when the light has interfered constructively -- where a crest of a wave meets a crest. The dark regions show destructive interference -- a crest meets a trough.
A formula linking the slit separation s, wavelength of light λ, distance from the slits to the screen D, and fringe width (the distance between the centres of the observed bands of light -- x) exists:
- λ / s = x / D
This is only an approximation and depends on certain conditions.
It is possible to work out the wavelength of light using this equation and the above apparatus. If s and D are known and x is observed, then λ can be easily calculated.
External links, references, and resources
Last updated: 05-07-2005 11:28:53
Last updated: 05-13-2005 07:56:04