The Original Double Slit Experiment

Light is so common that we rarely think about what it really is. But just over two hundred years ago, a groundbreaking experiment answered the question that had occupied physicists for centuries. Is light made up of waves or particles?

The experiment was conducted by Thomas Young and is known as Young’s Double Slit Experiment. This famous experiment is actually a simplification of a series of experiments on light conducted by Young. In a completely darkened room, Young allowed a thin beam of sunlight to pass through an aperture on his window and onto two narrow, closely spaced openings (the double slit). This sunlight then cast a shadow onto the wall behind the apparatus. Young found that the light diffracted as it passed through the slits, and then interfered with itself, created a series of light and dark spots. Since the sunlight consists of all colours of the rainbow, these colours were also visible in the projected spots. Young concluded that light consist of waves and not particles since only waves were known to diffract and interfere in exactly the manner that light did in his experiment.

The way I have always seen this experiment performed is with a laser and a manufactured double slit but since the experiment was conducted in 1801 I have always thought that it should be possible to recreate the experiment using sunlight and household materials. That is basically what I did here. I will show the interference pattern I observed with my homemade double slit on 2Veritasium but I chose to use a manufactured double slit here to ensure that the pattern was impressive for observers at the beach.

Special thanks to Henry, Brady, and Rupert for their cameos, Glen for filming and Josh for helping create the apparatus. Thanks also to the Royal Society for allowing us to view the original manuscript of Young’s lecture and the University of Sydney for lending the double slits.

Music by Kevin Mcleod ( Danse Macabre, Scissors


47 thoughts on “The Original Double Slit Experiment

  1. But isn’t the light source (the sun) not just shining down through the slits from one section of the sun, but light is coming from billions and billions of different areas of the sun? I mean, the suns light isn’t like a laser, coming from just the middle of the sun but from all over the sun. As such the light (protons, I think) coming through the slits are totally independent of the other light coming through the same slit.
    and if you have a round object shining through a narrow slit you will see that round object come out the other side of the narrow slit. With 2 narrow slits you will also see a mirror image of the round object just brighter in the middle and spreading out as shown.

    Can you get this wave pattern with just one slit? If not, I think it’s safe to say the only reason you get this wave pattern with two slits is because as the round light source is shining through the slits the middle of the light source is rotating (switching back and forth) which slit to go through (there has to be a point in the middle of the light source where the protons have the probability of going through either slit and a proton following another proton does in fact go through a different slit than the proton in front of it) and this causes the middle part of the wave pattern to be the brightest.

  2. Sunlight is a mix of photons traveling at mixed polarities and mixed frequencies. The wavy behavior you observe is an artifact of quantum mechanics, that boils down to random uncertainty of the of how the light acts when it passes close to a slit border. This results in a probability distribution of light, that then adds up with the light from the other slit, making up the observed interference pattern. (Photons don't really interfere, it is just called an interference pattern). To summarize, light is particles, but their behavior can be confused, and often explained with wave theory.

  3. The light is a particle no doubt our eyes would not be able to see it otherwise we don't have antennas.
    And when you it's not so difficult to understand the mechanics one you understand all the data that is out here.

  4. So I'm a construction worker. So you put another double slit in the most frequent band and so on In my mind an infinite chain would make a fractal or does it break down at some point

  5. Sorry but your video start like two theories competing and one that won through experiment… But it is more than that… Your video is like a conclusion that light is a wave and you demonstrate by the double slit experiment…
    But they made some other experiments that show that particles may act as a particle and as a wave at the same time. They shown that because they tried to spot the moment when the "wave/particule" pass the slit because a particule can only pass one slit while a wave will hit both slits and what they discover is that when they measure the particle location, it behave like a particle and otherwise it behave like a wave…

  6. What is a photon? I know it behaves simulataneously as a particle and wave. But I'm struggling to understand it. I'm trying to explain the double slit experiment with photons, instead of just looking at it from a wave nature of light. Do you guys have any idea or sources on this?

  7. Great video, it’s good to see people enthusiastic to learn. But Young’s handwritten notes are relics and shouldn’t be touched with bare hands!

  8. Excellent video. But please do not touch important historic old documents with your bare hands, use gloves and something to cover you mouth to prevent the documents from receiving saliva micro droplets while you read them loud at such a short distance from them.

  9. This interference is due to two narrow spherical holes,instead of two holes when we use slits we see the interference pattern in the form of dark and bright bands.😊

  10. La luz también es una partícula, si le hubiera dicho que la luz es partícula y onda al mismo tiempo y que depende de si hay un observador o no, hubieran quedado aun mas facinados

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