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Demonstration: Young's double slit experiment

Thomas Young's 1801 double slit experiment can be easily shown using a laser.

Some distance between the slits and the screen is needed to allow the waves to constructively or destructively interfere with each other causing the light bands that show that light is indeed a wave.


Day 1. Genesis 1:3 – And God said, “Let there be light,” and there was light.

~AD1380? Sayana (c. 1315-1387). Max Müller's translation of Sayana's commentary on the Rig Veda states: "Thus it is remembered: [O Sun] you who traverse 2202 yojanas in half a nimesa." This corresponds to a speed of about 302,073 km/s, which is close to the speed of light (299,792 km/s).

1660s. Pierre Gassendi proposed a particle theory of light while Robert Hooke published a wave theory of light.

1675. Isaac Newton published Hypothesis of Light.

1678. Christian Huygens thought light was a wave (Treatise on Light published in 1690).

1704. Newton published his final version of his theory in Opticks. He said light was particles. (We now call these particles photons). Newton's reputation helped this theory dominate during 18th century (the 1700s).

1801 (or maybe as late as 1805). Thomas Young's double slit experiment, which was such good evidence for wave theory of light that the wave theory held sway during most of the 19th century (the 1800s). Thomas Young also came up with the idea that different colours were different wavelengths of light. Some think Thomas Young was the last person to "know everything." As well as his scientific work he was also the first person (in recent history, obviously) to translate Egyptian hieroglyphs, using the Rosetta Stone.

1845. Michael Faraday discovered that the angle of a polarised light beam could be changed by a magnetic field, so light and magnetism are connected somehow. In 1847 he proposed that light is a self-propagating high-frequency electromagnetic vibration, which (unlike sound) does not need a medium.

Faraday's work got James Clerk Maxwell into light in a big way, and he figured out all sorts of things, such as the speed of light is constant. Unfortunately that contradicted the laws of motion which said that all speeds were relative to the observer.

1905. Albert Einstein figured out that light has a dual wave-particle nature and explained the photoelectric effect, where electrons get liberated after being hit by light, making solar panels possible. If the photoelectric effect had been caused by a wave the maximum energy of electrons dislodged would have proportional to the intensity of the incident light. Instead they were also proportional to the frequency. So particles were back. Einstein also discovered stimulated emission while researching the photoelectric effect.

Einstein also figured out the solution to the speed of light being constant, and physics students have been muttering about relativity ever since (special theory in 1905, general theory in 1914).

1915. After ten years of work Robert Andrews Millikan experimentally confirmed Einstein's work, all the while trying to disprove Einstein. Einstein got the Nobel Prize for it in 1921 and Millikan got it in 1923, partly for that and partly for his famous 1909 oil-drop experiment in which he measured the charge of a single electron.

1974. Two slit experiment done with one electron at a time and it STILL worked.

Since then the double slit experiment has been performed with other particles as large as buckyballs (C60) and it still works even one buckyball at a time.

Religious beliefs of above historical figures

  • Pierre Gassendi – Catholic priest and astronomer/astrologer.
  • Isaac Newton – believed in God but not in the Trinity. He has also been accused of astrology and numerology.
  • Thomas Young – Quaker (a Christian sect).
  • Michael Faraday – devout Christian.
  • James Clerk Maxwell – devout Christian.
  • Albert Einstein – humanist, non-observant Jew (but attended a Catholic elementary school).
  • Robert Millikan – Christian.

Note that simply because many of these scientists were Christians does not mean that they would be happy attending our own churches. They were mostly very much more conservative than we are.

Rainbows and other atmospheric phenomena

No two people see exactly the same bow because they depend on the relative positions of sun, viewer, and the water drops or ice crystals causing the refraction of light.


A large continous bow 40°-42° out from its centre, the point directly opposite the sun from the viewer (the antisolar point).

A secondary bow at 50°-53° can also form outside the main one, with its colours reversed (red on the inside). This is not the same as supernumary bands, shown at far right. Supernumary rainbows was evidence that light is a wave (proved by Thomas Young).

A fogbow is an "ordinary" rainbow that appears in fog, except that it is colourless (or yellowish) thanks to the small size of the water droplets. It is also called a sea dog.

Click on the pictures for larger versions. Photographs copyright © 2004-2006 Ian Mander, except where otherwise noted.

A full rainbow is quite hard to photograph – at 84° across it requires a very wide lens, and why my pic has been joined. If the sun is higher than 42° the viewer would need to be on a mountain to see any bow.
Supernumary colour bands.
Supernumary bands are clearest when the water drops are small and the same size. Photograph © Andrew Dunn, 27 September 2005. Photo retrieved from Wikimedia here.


A small continuous bow surrounding the point directly opposite the sun, when fog is beneath you but not above you (between you and the sun) and so very unusual to see without a shadow. Their size varies from 5° to 20° depending on the size of the drops. Commonly seen from aircraft flying over clouds.



Sun dog or moon dog.

A discontinous bright patch 22° away from sun on the horizontal or the vertical. Made by light passing through tiny (horizontal) hexagonal plate shaped ice crystals in thin cirrus clouds. They are sometimes coloured (apparently if the ice crystals are all about the same size). Also known as a parhelion (plural parhelia) or mock sun, or paraselene or mock moon.  
The sun is out of picture to the right.

Icebow or halo.

A continuous bow, sometimes coloured, surrounding the sun or moon, also at 22° out. They are caused by pencil shaped ice crystals, which might be at any angle, and so diffract light at any angle, making a full bow.


Neon lights.

When covering Module 13 – Nuclear force we discussed electron shells and neon and I mentioned we'd get back to it. Neon is a inert gas because its outermost electron layer is full. This means it doesn't easily interact with other atoms. However, one of those electrons in the outermost shell can be excited into a higher energy electron shell. When it falls back to its normal shell it gives off a photon with a suitable amount of energy for us to be able to see it. Hence we get a neon light. Simple.

De Broglie wavelength.

On the subject of single electrons behaving as waves, we could add another event to 1923: Louis de Broglie came up with the idea that matter actually consists of waves. He assumed that any particle (electron, atom, bowling ball, whatever) had a "wavelength" that was equal to Planck's constant divided by its momentum. He knew that the momentum and the wavelength of a photon are related in this way. (Photons don't have mass, but they do have energy, and so have momentum.) And so we have this interesting quote:

"Physical objects from quarks to planets have wavelike attributes. The quantum nature of a bowling ball, unfortunately, is not manifest since its equivalent quantum (or de Broglie) wavelength is so tiny that interference effects (for example, the left part of the ball negating the right part of the ball) cannot be detected in a practical experiment." – American Institute of Physics Bulletin of Physics News Number 453 (summarising Arndt et al, Nature, 14 October 1999).

This web site also has easy-to-understand information about it:

According to de Broglie, the wavelength is equal to Planck's constant divided by the object's momentum; Planck's constant is very, very, very tiny, and the momentum of a bowling ball, relatively speaking, is huge. If you had a bowling ball with a mass of, say, one kilogram, moving at one meter per second, its wavelength would be about a septillionth of a nanometer. This is so ridiculously small compared to the size of the bowling ball itself that you'd never notice any wavelike stuff going on; that's why we can generally ignore the effects of quantum mechanics when we're talking about everyday objects. It's only at the molecular or atomic level that the waves begin to be large enough (compared to the size of an atom) to have a noticeable effect.


Java-based wave tank. Sonic boom (see Module 14), two slit experiment etc.

Some more pictures of rainbows, halos and sundogs. This pic is an impressive set of halos and sundogs at the south pole.

The Atmospheric Optics site has great photos of a whole bunch of interesting bows and halos.