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Young's double-slit experiment shows that matter and energy have properties of both waves and particles. The base of this experiment involves a light source being shined on a thin plate that has two parallel slits, and the light that is passing through the slits is observed on a screen behind this plate. Since light moves as a wave, the light waves passing through the slits interfere with each other in a wavelike pattern. This causes bright and dark bands to show up, which would not occur if light were composed strictly of particles. However this experment showed that light is absorbed as discrete particles, which helped to establish the principle of wave-particle duality.
II. Experimental Procedure
Young shone sunlight through a thin card containing a single slit, which served to diffract the light beam into a spherical wave. The wave then passed through another screen behind the first one, this time with two slits. The two slits diffracted the single wave into two interfering waves.1 The experiment is shown in the diagram above Light-colored bands corresponded to regions where the two waves from each slit were in phase, causing constructive interference while dark bands, regions where the two waves were exactly out of phase exhibited regions of destructive interference. 2 The light and dark bands correspond to the probability of the photons being in the corresponding area. The electron was likely to be in the light banded areas, and unlikely to arrive in the dark areas.
III. History of the Wave-Particle Duality of Light
In the 1600’s, scientists thought light was a particle, with a small mass, and a very high speed. This explained many properties of light, including the photoelectric effect. They thought that light could not travel as a wave, despite the particle theory being unable to explain the wave interference pattern shown in Young’s double slit experiment. If this theory were to have been true, then the light would pass straight through the slits, and make two exact slit-shaped lines on the screen.5
In the early 1800s, many scientists still believed in Newton’s corpuscular theory of light, which treated light as particles. It was not until early in the 19th century that Thomas Young was able to perform his experiment to show the wavelike properties of light. He reasoned that if light consisted of waves, then it should behave as would ripples colliding on a body of water. He believed he could show that light passing through two slits caused a wave interference pattern due to the fact that when separate waves meet in phase, they add together to produce a wave of higher amplitude, whereas if they meet out of phase, the two waves cancel each other out, forming a flat region.2 However, no scientific institutions would take him seriously, even after he successfully performed his experiment. However, Young’s experiment did not gather any support until 1818, when Augustin Fresnel showed a mathematical form of wave theory, that people began to listen to Young and believe that light did indeed act as a wave. 6
Not until Einstein suggested the theory of wave-particle duality that the true form of light emerged. He rationalized that light, and indeed all particles have both particle-like and wavelike properties, and that in particles other than light, it was simply too miniscule of a wave to actually measure on any scale. It was known that it was impossible for light to cause the wave interference pattern shown in the double slit experiment without having some wavelike properties, however also that light had to have some particle like properties, because it can transfer some of its energy to an electron in the photoelectric effect.7
In the mid-1670's Dutch Physicist Christiaan Huygens came up with novel ideas that later proved to be of great importance to physics and also to Young's Double Slit experiment. Huygens’ Principle of wave analysis states that when you have a wave, you can view the “edge” of the wave as actually creating many circular waves. These waves usually conglomerate together to continue the wave propagation, and the wave front is shown below12:
Huygens’ principle of diffraction.
Wave front and illustration of Huygens' Principle.
Huygens was the first person to explain how wave theory can also account for the laws of geometric optics. Most scientists in the 1670s didn’t even take the slightest notice of him, but his work was later rediscovered after the eventual triumph of wave theory. Huygens’ principle is convenient for calculating wave phenomena. However, it is only applicable as a classical wave-optics model. In the double-slit experiment, Young built off of Huygens’ work and utilized a single laser beam to illuminate two slits, successfully showing that matter and energy have properties of both waves and particles.11
In 1909, Sir Geoffrey Ingram Taylor managed to lower the level of emitted light until only one photon was traveling at a time, and recreated Young's Double Slit experiment one single electron at a time. Remarkably, a wave interference pattern still occurred. This suggested that the photon had somehow managed to interfere with itself, by simultaneously going through both slits at once. This can be explained by understanding that each photon has it's own wavefront that passes through each slit, causing the probability of where each photon will land to change.
IV. Wave Nature of Light
When a single, monochromatic wave is diffracted into two separate waves, interference occurs as a result of collision between the two waves. When the pattern is reflected on a screen, it displays alternating bands of light and dark regions. The light regions correspond to constructive interference, or where the wave crests meet with the same frequency, while the dark regions correspond to destructive interference, where the crest of one wave and the trough of another cross paths. This concept could be explained geometrically.3
Observe the images below:
Waves out of phase Distance variation among waves of light
Consider light hitting point 1 on screen B. The ray from slit Q must travel considerable more distance than that from slit P, thereby producing a phase lag relative to the light from slit P. The opposite is true with point3. These points correspond to dark regions of destructive interference, because the difference in the the time it took for the wave to travel that distance caused the waves to be exactly out of phase. On the other hand, the light beams from the two slits travel the same distance to reach point 2, so the waves arrive there with the same phase, since they both traveled the same distance. This produces light regions of constructive interference.2 The waves display constructive interference when the waves line up. This can occur at every change in time x or y, so at different points on B, there would be many constructive interference points. When the waves are not in phase every change in time x or y, then at different points on B, there would be numerous destructive interference points.
V: Particle Nature of Light
When the wave is shined through the slits, contrary to the particle nature of light, the light beams hit the screen at once, as opposed to hitting in many different small projectiles. The waves, all moving at the same speed would all travel together, causing wave interference, and reach the screen simultaneously at all the points, as opposed to the machine gun-like hail of photons. In this case, the wave and particle characteristics both prove to be true. The wave nature of light is demonstrated through the bands formed on a screen, while the particle nature of light is exhibited by the numerous tiny specks that constitute those bands.
Thomas Young - Young's Double Slit Experiment
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