The Original Double Slit Experiment

19 Feb 201307:39


TLDRThe video script delves into the nature of light, exploring its dual properties as both wave and particle. It starts with a philosophical inquiry about light and its relation to auras, then transitions into a historical overview of scientific debates, highlighting Newton's corpuscular theory and Huygens' wave theory. The pivotal moment is the recreation of Thomas Young's double slit experiment, which demonstrated light's wave-like behavior through interference patterns. The experiment's outcome, with its colorful fringes, underscores the importance of wavelength in color formation. The video concludes with an invitation for viewers to ponder the experiment's implications, emphasizing the wave-particle duality of light.


  • 🤔 The nature of light has been a subject of philosophical and scientific inquiry for centuries.
  • 🌞 Light is often associated with brightness and the presence of visible spectra.
  • 🎨 The difference between blue light and red light lies in their color, which is determined by their wavelengths.
  • 👀 Human perception of light is through the eyes, which translates the light into visual information.
  • 🌈 The concept of auras, though not scientifically proven, is a belief that each person emits a unique light.
  • 🔬 Sir Isaac Newton proposed that light consists of particles or corpuscles.
  • 🌊 Huygens, on the other hand, theorized that light behaves as a wave.
  • 🧪 Thomas Young's double-slit experiment provided strong evidence for the wave nature of light.
  • 📜 Young's handwritten notes from 1803 detailed his observations of light passing through narrow slits and creating interference patterns.
  • 🌈 The interference pattern produced by the double-slit experiment shows constructive and destructive interference, leading to a series of bright and dark fringes.
  • 💡 The round blobs observed in the experiment are due to the diffraction of light, which is a property of waves, further supporting the wave theory of light.

Q & A

  • What is the fundamental question addressed in the transcript?

    -The fundamental question addressed in the transcript is 'What is light?', exploring its nature and properties through discussions and experiments.

  • What did Newton propose about light in the late 1600s?

    -Newton proposed that light was a stream of particles or corpuscles, as stated in his treatise, Opticks.

  • How did Huygens contrast with Newton's view on light?

    -Huygens proposed that light was a wave, contrasting with Newton's particle theory.

  • What experiment was conducted to settle the debate between Newton's and Huygens' theories?

    -Thomas Young's double slit experiment settled the debate, demonstrating that light behaves as a wave due to the interference patterns produced.

  • What did the speaker find in the vault underneath the Royal Society in London?

    -The speaker found Thomas Young's handwritten notes from 1803, detailing his observations and experiments with light.

  • How does the double slit experiment demonstrate the wave nature of light?

    -The double slit experiment shows the wave nature of light through the interference patterns created when light waves from the two slits interact, producing bright and dark spots on the observation screen.

  • What is the principle behind the interference patterns observed in the double slit experiment?

    -The principle is that when light waves from the two slits meet, they can either constructively interfere (peaks meet peaks, troughs meet troughs) to produce bright spots, or destructively interfere (peak meets trough) to produce dark spots, due to the superposition of waves.

  • Why do different colors appear in the interference pattern of the double slit experiment with sunlight?

    -Different colors appear because sunlight is composed of many different colors, each with a different wavelength. These wavelengths interact with the slits at slightly different points, causing the rainbow effect as one moves away from the central maximum.

  • What is the significance of the observed round blobs of light on the bottom of the box in the experiment?

    -The round blobs of light indicate the diffraction of light, which is a wave property. This phenomenon occurs because light waves bend around obstacles and spread out as they pass through the narrow slits.

  • How does the conversation in the transcript contribute to the understanding of light's properties?

    -The conversation provides a real-life, relatable context to the scientific concepts being discussed. It helps demystify the nature of light by relating it to everyday experiences and observations, making the scientific principles more accessible and understandable to the average person.

  • What was the conclusion reached by the scientific community after Young's double slit experiment?

    -The scientific community concluded that light must be a wave, as the results of the double slit experiment could not be explained by a particle theory of light.



💡 The Nature of Light and Perception

This paragraph delves into the philosophical and scientific inquiry about the nature of light. It begins with a series of rhetorical questions, highlighting the complexity of defining light. The discussion moves on to the concept of auras, suggesting that they are a form of light that illuminates a space. The conversation then touches on the differences between blue and red light, attributing the distinction to their color. The explanation of how light enters our eyes and enables us to perceive our surroundings is juxtaposed with the idea of auras, which are suggested to be a type of light visible to some but not to others. The paragraph also explores the challenges of explaining light to someone who cannot see, drawing a parallel between the limitations of human perception and the vastness of the observable universe. The historical context is provided by mentioning Isaac Newton's and Christiaan Huygens' contrasting theories on light as particles or waves, respectively. The narrative then shifts to the famous double-slit experiment conducted by Thomas Young, which played a pivotal role in settling the wave-particle duality of light. The experiment's methodology and findings are described in detail, emphasizing the simplicity and profound implications of the experiment.


🌈 Wave-Particle Duality and Color Theory

This paragraph continues the exploration of light's properties, focusing on the wave-particle duality and the phenomenon of interference. It begins by explaining how constructive and destructive interference create patterns of bright and dark spots, similar to those observed in Young's double-slit experiment. The analogy of water ripples is used to illustrate how waves interact, leading to a better understanding of the interference patterns observed with light. The paragraph then discusses the composition of sunlight, highlighting that its various colors are due to different wavelengths. The interaction of these wavelengths in the double-slit experiment results in the colorful patterns seen, a direct consequence of their varying lengths. The summary emphasizes the beauty and complexity of light and color, and how our perception of these elements is shaped by their underlying physical properties. The paragraph concludes with a reflection on the enlightening nature of understanding these scientific concepts.




Light is a form of electromagnetic radiation that is visible to the human eye. It is often characterized by its brightness and color, and is a fundamental aspect of our perception of the world. In the video, light is explored not just as a source of illumination but as a subject of scientific inquiry, with discussions on its nature as either a particle or a wave, as well as its role in the phenomenon of auras and color perception.


An aura, in the context of the video, refers to a hypothetical luminous emanation surrounding living beings, often associated with spiritual or psychic phenomena. While the scientific basis for auras is debated, they are described in the video as being made of light that can illuminate a room and are connected to the concept of individual energy fields.


Color is the visual perception of different wavelengths of light, which are absorbed, reflected, or transmitted by objects and detected by the photoreceptor cells in the human eye. In the video, color is discussed in relation to the properties of light, particularly in the context of the double-slit experiment, where the interference pattern produced by light results in a spectrum of colors, illustrating the concept of different wavelengths corresponding to different colors.

💡Double-slit Experiment

The double-slit experiment is a classic physics demonstration that shows the wave-particle duality of light. In the experiment, light passing through two closely spaced slits creates an interference pattern on a screen behind the slits, demonstrating the wave-like behavior of light through the interference of light waves. This experiment was pivotal in the development of quantum mechanics and our understanding of the nature of light.


Wavelength is the spatial period of a wave— the distance over which a wave's form repeats. In the context of light, different wavelengths correspond to different colors, with longer wavelengths typically associated with red light and shorter wavelengths with blue light. The video script discusses how the different colors in the double-slit experiment are due to the various wavelengths of light interfering with each other.


Interference is a phenomenon in wave physics where two waves meet and combine to form a new wave pattern. This can result in constructive interference, where the waves amplify each other, or destructive interference, where they cancel each other out. In the video, the concept of interference is central to explaining the double-slit experiment and the resulting pattern of light and dark bands on the screen.


Isaac Newton was an English mathematician, physicist, astronomer, and author who is widely recognized as one of the most influential scientists of all time. In the video, Newton's contribution to the understanding of light is mentioned through his proposal that light consists of particles or corpuscles, as outlined in his work, Opticks.


Christiaan Huygens was a Dutch mathematician, astronomer, and physicist who made significant contributions to the field of optics. In the video, Huygens is noted for his proposal that light is a wave, which was in contrast to Newton's particle theory of light. This wave theory was eventually supported by the results of the double-slit experiment.

💡Thomas Young

Thomas Young was an English polymath who made significant contributions to physics, linguistics, and other fields. In the video, Young is credited with performing the double-slit experiment that demonstrated the wave nature of light, which was a pivotal moment in the history of physics and helped to resolve the debate between the particle and wave theories of light.

💡Constructive Interference

Constructive interference occurs when two waves meet in such a way that their peaks align with each other's peaks and their troughs with each other's troughs, resulting in a wave with a greater amplitude than the individual waves. This phenomenon is a key aspect of wave behavior and is illustrated in the video through the bright spots observed in the double-slit experiment where light waves from the two slits combine to enhance each other's intensity.

💡Destructive Interference

Destructive interference occurs when two waves meet such that the peak of one wave aligns with the trough of another, causing the waves to cancel each other out and resulting in no wave or a wave with reduced amplitude. In the context of the video, this is observed as dark spots in the double-slit experiment where light waves from the two slits interfere in a way that reduces or eliminates the light intensity at certain points.


The philosophical question 'What is light?' is explored, highlighting the enduring nature of this inquiry.

The concept of auras as a form of light is introduced, suggesting a metaphysical aspect to the nature of light.

The distinction between blue light and red light is discussed, emphasizing the role of color in the perception of light.

The historical debate between Newton's corpuscular theory and Huygens' wave theory is mentioned, providing a context for the scientific evolution of understanding light.

Thomas Young's double slit experiment is described as the pivotal test that helped settle the wave-particle duality of light.

The process of recreating Young's experiment using sunlight and accessing Young's original notes is detailed, illustrating the experimental methodology.

The observation of colored fringes and parallel fringes in the shadow of a card is explained, demonstrating the interference pattern crucial to understanding light's wave nature.

A public demonstration of the double slit experiment in a box is conducted, showcasing the practical application of scientific principles.

The expectation of seeing two lines versus the actual observation of multiple dots is highlighted, emphasizing the counterintuitive nature of light's behavior.

The explanation of constructive and destructive interference is provided, linking the behavior of light waves to the observed patterns.

A water ripple analogy is used to illustrate the principle of wave interference, making the concept more accessible.

The impact of different wavelengths on color production is discussed, connecting the physics of light to everyday experiences.

The difference in color perception due to varying wavelengths is clarified, using the example of a red bin versus a green pot.

The observation of round blobs instead of slit shapes in the light pattern is noted, pointing to a phenomenon that requires further explanation.

The conclusion that light must be a wave based on the double slit experiment is stated, reflecting a significant scientific consensus.

The experiment's ability to enlighten and amaze participants is emphasized, showcasing the transformative power of scientific discovery.



What is light?


What is light? Light is... light is... what is light? That's a good question, isn't it? What is light?


Isn't it an element?


Light is brightness, I guess.


- We have auras? - We all have auras.


- Which are light? - Yes, they are.


It lights up the room, it makes it... not dark.


- What's the difference between blue light and red light? - The color.


It goes in your eyes and then you see stuff.


The range from white to red to orange to green, it's like the chakras of your body.


- Can you see my aura? - No, not particularly right now.


- Is it too bright out? - It's very sunny out here today.


- Does that make it harder to see someone's aura? - Not necessarily.


If I was to explain it to a blind person, I'd... It would be... It would be the difference... You see nothing whatsoever as a blind person, whereas I see things in front of me.


To be fair, the question of what light is, is not an easy one. For centuries, the greatest minds in science debated this issue.


In the late 1600s, Newton proposed that light was a stream of particles or corpuscles. He proposed this in his treatise, Opticks.


But at the same time, a dutch physicist named Huygens proposed that light was a wave.


And this debate raged on until it was settled by the experiment I've recreated today, Thomas Young's double slit experiment.


To make sure I got the experiment right, I went to the original source. With a help of Brady Haran, I managed to get into the vault, underneath the Royal Society in London.


We're in!


There, I found Thomas Young's handwritten notes from 1803.


I brought into the sunbeam a slip of card, about one-thirtieth of an inch in breadth,


and observed its shadow, either on the wall or on other cards held at different distances.


Besides the fringes of color on each side of the shadow,


the shadow itself was divided by similar parallel fringes, of smaller dimensions.




This is an experiment so simple that you could make it at home, and yet so fiddly that I've never seen it before done with sunlight.


- I was thinking about doing it in a box, like a fridge box. - And you could take it out on the street.


Taking it out on the street.


Could I possibly interview you guys for about a minute?


We're doing a science experiment.


What I have here is an empty box, and this is a little eye piece where we can look in, and this is a hole.


And I'm gonna place this slide above that hole, and if you look closely, you'll see that there is two openings, very narrow openings side-by-side. It's a double slit.


Now, before we have a look, we need to tilt it towards the sun a little bit, so... we want the sun to hit this double slit directly.


- What are we gonna see on the bottom of the box? -The obvious thing you think you're gonna see is you're gonna see two lines.


Two lines on the bottom of the box.


Two broad-bands.


- Two little lines. - Yeah.


I think it will be one... one line, sort of two.


I can expect to see the whole box lit up.


- Probably a kaleidoscope, of some sort. - A bunch of colors.


Probably, yeah.


Rainbow? Different colors?


There, have a look.


You expected to see kind of one line - is that what you see?




I see dots.


How many?


It's one circle.


Oh, there's one.. there's one in the middle, the strongest, two either side.


The two on the outside are multicolored, and the one in the middle... is just white.


It looks kind of a rainbow.


The rainbow of color as well.


Quite a few colors and lots of little dots.


And there are more dots appearing.


I think I can even see more dots spreading along. Yeah, that's amazing.


Yeah, I can see tons of dots now. Not tons, but I can see dots spreading across that way.


- On either side? - Yeah, definitely.


- Isn't that amazing? - Yeah, that's incredible.


- And that's just nothing else apart from... - Two slits.


That's incredible.


But all we're doing is we're putting light through two very narrow slits side-by-side, so how does this make any sense?


There's some kind of principle involved in it that the average person's not familiar with. That's the only explanation.


I'm really confused by it, but I'd like to find out why.


People were debating: Is light a wave or is it made of particles?


So what causes that?


Well, if light were behaving as particles, you would expecting to go through each slit and just produce a bright spot underneath,


so we would see two bright spots under the bottom of the box.


But, if light is behaving as waves, then the wave from one slit can interact with the waves from the other slit.


I've got a demonstration here on a little pond, where we can see this with water waves.


I have two sources of ripples, which are basically like the two slits, when I create ripples with a single source, they travel out with circular wave front, nothing particularly surprising there.


But, if I add a second source of ripples, then we start getting an interesting pattern.


This pattern is created by the ripples from the two sources interacting with each other.


Where they meet up peaks with peeks and troughs with troughs, the amplitude of the wave is increased, that's what we call constructive interference.


But if the peak from one wave meets up with the trough from the other, then we get destructive interference and there's basically no wave there.


And this is exactly what was happening with the light.


When the light from one slit met up peaks with peaks and troughs with troughs, they constructively interfered and produced a bright spot.


But, if the trough from the wave from one slit met up with the peak of the wave from the other slit, they would destructively interfere, and you wouldn't see any light there.


It's light canceling itself out.


- This is basically the same is like having two drops of waterfall in a swimming pool, - That's right,


- exactly the same pattern. - and then they're going overlap


As this ripple overlaps with those ripples, down the bottom, you get a series of...


you get like a bright spot... and then a dark spot, then a bright spot, then a dark spot, then a bright spot.


Now there is a slight complication, which is that sunlight is composed of many different colors, and they have different wave lengths.


So, obviously, they're gonna meet up at slightly different points, and that's what caused the rainbowing effects as we go further from the central maximum.


You saw the ones to the right were slightly colored, that's because the reds are gonna meet up at different places than the blues.


And that's all that makes color differences, is different wavelengths?


- Exactly. - That's amazing.


So the difference between—


So that red bin over there on the green pot it's just... I'm seeing that


- It's a different wavelength. - It's just different wavelength.


- And that's how we're bringing these beautiful colors all around us. - Exactly.


That's amazing, I'm amazed! Thanks, man!


Thank you!


I have been enlightened, literally.


Now, you may have noticed in that experiment that the light on the bottom of the box was not in the shape of slits, rather they were more kinda round blobs,


and I want to know why that is.


Can you write an answer for me in the comment section? And I'll give you a hint: one of the videos I've linked kind of is suggestive of the answer.


So convincing were the results of Young's double slit experiment that the scientific community concluded that light must be a wave, there is no way it could be a particle.

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