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.

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.

- 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.

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,

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.

- 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.