The Original Double Slit Experiment

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 the 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."

Wow. 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 eyepiece where we can look in, and this is a hole. I'm gonna place this slide above that hole, and if you look closely, you'll see that there are 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?

No! 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 expect it 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 a 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 peaks 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 as like having two drops of waterfall in a swimming pool,
• That's right,
• exactly the same pattern,
• and then they're going to 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 wavelengths. 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 kind of 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.