High School High School Physics and Mathematics TSS

A Crash Course in Quantum Mechanics: The Double-Slit Experiment

An introduction to some fundamental concepts of quantum mechanics and an explanation of the original and modern double-slit experiment.

No other subject stumps and challenges scientists as much as quantum mechanics. The branch of physics, which emerged around 1925, centers around the study of extremely small particles, such as photons and electrons. These particles seem to not follow many of the conclusions scientists have made about the world around us, resulting in interesting theories regarding what is really happening at such a small scale. Quantum mechanics is extremely complex, but in this series, I will try to provide some background and some overarching ideas about the subject.

Prior to the development of quantum mechanics, scientists believed that matter came in two forms: particles and wave-like matter (1). However, certain experiments proved that this concept was too simplistic. One famous one is the double slit experiment, first performed by Thomas Young in 1801.

When two in phase waves with an equal amplitude meet, there is constructive interference, meaning that one wave with double the amplitude is formed. When two out of phase waves with an equal amplitude meet, there is destructive interference, meaning that the two waves cancel each other out.


Image from Physics and Radio Electronics

Photons, which make up light, had long been classified as particles (although they were not given the name photons until 1926). This was largely due to Isaac Newton’s work. In 1704, Newton wrote a book called “Opticks” in which he put forth his “Light Particle Theory.” Young had suspicions that light was in fact a wave and he aimed to prove this. To do so, he devised an experiment with three screens. In the first screen, there was a single slit through which sunlight would pass through. In the second screen, there were two slits through which the light would again pass through, allowing the light to be diffracted, or spread out. The third screen contained a dark film which allowed Young to view the resulting light intensity distributions.


Image from Molecular Expressions

If light was truly wave-like in nature, then areas of constructive and destructive interference should be visible on the third screen. This was, in fact, what Young found. For each beam of light to travel to point A on the third screen, they must travel an equal distance, allowing the frequencies to be in phase with one another. The red bands on the third screen demonstrate constructive interference, areas where light waves combined to form waves with a larger amplitude. To travel to point B, one wave must travel further than the other, resulting in the frequencies being out of phase. The black bands on the screen demonstrate destructive interference, areas where light waves combined to form waves with a smaller or zero amplitude (2). Through this experiment, Young demonstrated the wave-like properties of light, but it took many years for his fellow scientists to accept his work.

This famous experiment has been reinvented to study individual particles. In another version of the double slit experiment, a similar setup is used but instead of sunlight, a stream of electrons (or photons) is fired at a screen. When a stream of electrons is fired, the electrons interfere with each other and form interference patterns, similar to the way light waves do. This demonstrates that individual electrons and photons do have wave-like properties. This concept of wave-particle duality, meaning that quantum systems have both properties of waves and properties of particles, is a fundamental principle of quantum mechanics.


Image from Wikipedia

However, what is strange is that this interference pattern is still formed if only one electron has been fired. At the slow rate of one electron per second, far too slow for the electrons to interfere with one another, the interference patterns still build up on the detector screen. This means that the particles must somehow be interfering with themselves (3).


Image from Steemit

There is no easy explanation for this phenomena, which introduces the idea of superposition. Superposition is the principle that a quantum system can exist in all possible realities at the same time, allowing the electron to interfere with itself (4). Scientists attempted to measure the actual locations of the electrons by placing detectors at each of the double slits. However, when they did this, the interference patterns disappeared and the electrons acted as particles. It was if measuring the location of the electron caused the superposition to collapse into a single reality. The idea of the measurement problem, wherein observation somehow disrupts the trajectories of quantum systems, is another central concept of quantum mechanics and one that makes the subject so intriguing to scientists.

Stay tuned for my next article in this series if you want to learn more about quantum mechanics!


(1) Norton, John D. “Origins of Quantum Theory.” Einstein for Everyone, 14 Apr. 2017, http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_origins/.

(2) Parry-Hill, Matthew, and Michael W. Davidson. “Thomas Young’s Double Slit Experiment.” Olympus, http://www.olympus-lifescience.com/en/microscope-resource/primer/java/doubleslitwavefronts/.

(3) “Two-Slit Experiments.” Star Formation, abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html.

(4) Hamer, Ashley. “The Double-Slit Experiment Cracked Reality Wide Open.” Curiosity.com, 11 Feb. 2017, curiosity.com/topics/the-double-slit-experiment-cracked-reality-wide-open-curiosity/.

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