[intro music] (host) Thanks to Curiosity Stream for supporting PBS Digital Studios.

Hey guys, Joe here.

What did people used to do when they were bored before we had all these devices to keep us occupied?

Like back in the old days, people had to be more creative.

Next time you feel like life's going too slow, why not put those idle hands to use and play with the fastest thing in the universe?

Here's three experiments that will show you how to bend light itself using nothing but your own two hands.

Try this, close one eye and look at something across the room, preferably with a nice straight edge or an interesting pattern.

Next, put your finger a few inches in front of your eye or a few centimeters.

It's your choice.

Now focus on that object and let your eye relax until both your finger and the background are slightly out of focus.

When you move your finger side to side, look what happens.

The light seems to bend around your finger.

For some people, it may bend toward your finger and for some it may bend away.

With a few tries, you should be able to do this pretty easily.

What's going on here, is your finger tugging at the fabric of the universe?

Well, a little bit.

Everything with mass warps space-time by some amount, but your finger isn't nearly massive enough for it to be noticeable unless you have a black hole in your finger, which would really suck, get it?

We'll explain that, but first, let's try another one.

For the next experiment, close one eye and look at something bright in the distance.

Not the sun, never look at the sun ever.

Just don't do it.

With one eye shut, slowly bring your two fingers together just in front of your eye.

Just before they touch, they seem to reach out and melt into one another.

Try it with two objects out on a sunny day.

Right before two shadows touch, they seem to push out and join.

That is cool.

You're basically a wizard now, bending light and bending dark.

Finally, for the third experiment, close one eye again and slowly bring two fingers together near your eye while watching the light through a slit in between them.

You should see a series of dark lines form.

As you move your hand back and forth, the pattern of those dark lines can change.

What's going on here?

How do these light-bending experiments work?

Well, let's science them and figure it out.

I'm gonna repeat the bending light with a camera as your eye.

I'm pointing it at a grid on my computer screen.

A key to why this works is that we keep the background slightly out of focus.

So what's going on here?

It has to do with what happens when light passes through a lens in your eye or in a camera.

Take any point out here in front of a lens.

Light reflects off that point, spreads out, passes through the lens and is bent toward a different point behind the lens.

The spot where the light comes together is called the focal point.

The lens in your eye and the camera work in different ways to focus images.

Your eye physically changes the shape of the lens while a camera moves it.

Both alter the point where light from different distances comes together.

When the lens is out of focus, light rays from a fixed point don't come together exactly on the sensor or our retina.

The resulting blurred image is formed by separate light rays passing through different parts of the lens.

If we insert something in between the object and the lens like a finger and block some of those rays, we've actually made part of the image disappear.

The center of the blurry area shifts to one side.

Now, a lens inverts the image and our brain or a camera flips whatever falls on the sensor.

So if we focus in front of the background, the object appears to move away from our finger.

If we focus behind the background, it seems to bend toward our finger.

This even works when we look at a thin object edge on.

At its most extreme, we can bend the grid so much that it breaks.

In reality, light at any point spreads out in a three dimensional cone, but this all works in the same way.

Putting something between our eye and an object narrows that cone of light.

We're not really bending light after all, we're blocking it.

Block enough light and you create a shadow like in our second experiment.

When the shadows of your two fingers or other objects mysteriously bulged toward each other when they got close.

This is called the shadow blister effect.

And to understand how it works, you need to understand the parts of a shadow.

The darkest part of a shadow where an object completely blocks the source of light is called the umbra.

This is the familiar dark part we typically call the shadow, but around the edges where only part of the light is blocked by an object, we have the hazier penumbra.

For your typical shadow cast by a bright light source like the sun, we don't see this hazy penumbra around the edge.

Our eyes can't make out the tiny difference in contrast.

But when two of these penumbra overlap, when two shadows get close enough together, they can block enough light to become visible or invisible and the shadows appear to bulge out.

So what about those dark bands we saw between our fingers?

Most of the explanations out there, including several in textbooks, say this is a demonstration of defraction, the interference of light waves after they pass through something like a narrow slit.

That would be really cool, but I'm not sure that's what's happening here and the experts I've asked about it agree for a few reasons.

First, for defraction to work, we usually need a coherent light source like a point of light or a laser that sends light waves out only in one direction.

We don't have that here.

Second, since different wavelengths or colors of light defract differently, white light should give us some strange colored bands, not just dark ones.

Finally, to see defraction through a single slit, it usually needs to be much narrower relative to visible light than the gap you can make with your fingers.

I think there's something else going on here, but I'm not sure what.

Maybe something due to the shape and size of the eye or the iris or even an illusion caused by how our brains detect edges or different levels of light.

Or maybe it's defraction after all even though I'm almost certain that it's not I think.

I want you to try this and do some experiments of your own.

Let me know what you think is going on and why.

I especially wanna hear from you if you're an optical physicist or a neuroscientist or something like that, but everyone is welcome to do some science on this.

We can solve it.

You can learn a lot from a little boredom.

Many great scientists, including Einstein, made time to be bored to let their minds wander.

One of Einstein's famous thought experiments led to his theory of general relativity.

This predicted a different kind of light bending called gravitational lensing where massive objects warped space-time enough to actually bend light from a distant source.

This idea, may be inspired by boredom, was observed in a 1919 solar eclipse and in later observations like Einstein's cross.

Bending of light from gravitational lens has even offered experimental support for the existence of matter we can't see, dark matter.

Often when we're bored, our reflex is to find something to do with these things.

But next time you find yourself unconsciously picking up one of these things, remember that a couple idle hands can inspire awesome things.

Stay curious.

[mock dog sounds] [people laughing] Huh.

So being bored can be a good way to inspire curiosity.

Speaking of curiosity, thank you to Curiosity Stream for supporting PBS Digital Studios.

Curiosity Stream is a subscription streaming service that offers documentaries and non-fiction titles from some of the world's best filmmakers, including exclusive originals.

I've been watching "Dream the Future," a 19-part series that asks leading visionaries to predict what life will be like in 2050.

It's exclusive to Curiosity Stream, and it's narrated by Sigourney Weaver.

The first ten episodes are streaming right now.

You can get unlimited access today, and for our audience, the first 60 days are free if you sign up at CuriosityStream.com/smart and use the promo code "smart" during the sign-up process.

Pow!

Accessibility provided by the U.S. Department of Education.