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.

[crazynfreaky]

we need more people like the yellow shirt guy. he's so enthusiastic about learning. its not a bad thing if you don't know things, rather it is when you don't want to learn.

[TheLuckymod]

That man was so happy at having learned something new. Wonderful.

[arnabsinha]

The guy in yellow has a brilliant spirit...star of the video!

[chrismccolm]

I like how the guy in the yellow was truly thankful for being taught this. Brian Greene blew my mind with this many years ago, and then I was amazed yet again at this incredible thing. Great video!!

[kirkhenry]

The man who was in the yellow shirt was amazed at learning something new, and it changed his perception. The genuine, legitimate joy on his face makes me feel so warm and happy. Outstanding video. Also the pure joy of teaching from the host is also a beautiful thing. Thank you so much, you made my day :)

[LouisXIII]

"What's the difference between blue light and red light?"
"The color."
*And so he went on winning the Nobel prize*

[drd]

"There's some kind of principle involved..that the average person's not familiar with"
Explanation I've heard on You Tube

[poonamsrivastava]

I learnt YDSE in my school but due to the fact that we had to learn various formulae (like amplitude, intensity, path difference, phase difference, fringe width etc.), I almost forgot how beautiful this experiment actually was.

[camelCaseFTW]

the yellow shirt dude just got his life changed

[bradyrobbins]

Me over here crying about the yellow t-shirt guy’s epiphany. Learning is truly beautiful. Coming to understand the universe is powerful.

[ndsheep]

"What's the difference between blue light and red light"
"The color"
Well he's not wrong...

[natalieelisabeth]

The guys reaction in the yellow shirt and blue hat- literally made my day! It’s so rare to observe someone genuinely experiencing amazement. It really is one of the purest forms of joy. So human. So awesome:)

[Xuevium]

Legend says that the yellow shirt guy is still in the park thinking about it to this day.

[ilovepizza]

i feel like the yellow shirt guy went to his mates after this and yelled "ITS WAVELENGTHS MATES!!! ITS THE BLOODY WAVELENGTHS!!"

[sudhanshurai]

We need science students who are like the Yellow T shirt guy....

[kidv]

There's something so pure and wholesome to see everyone genuinely wanting to understand,

[uilsoum]

To make sure i got the experiment right, i went to the original source. I resurrected thomas young

[maruftim]

How would you explain light to a blind person?

"I see, you don't."

[suyash]

Einstein: light is a particle.
Young: no it's a wave.
Old lady: isn't it an element.

[shermanhoman]

And here I am, ten years after this was posted, it is still the best explanation of the double slit experiment ever! Now I need to find the episode where you conclude that light also has to be a particle. And then the one where the existence of a photon is dependent on an object it "hits".