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The Physics of Invisibility Cloaks

6 minutes

(man) Every morning, you see your reflection in the mirror.

(woman) Every summer, your friend looks squiggly in the bottom of the pool. Can this help us make invisibility cloaks? Boo. What? Or explain the shiny wings of a butterfly.

(man) The answer is photonics, which is all the crazy ways to mess with light!

(woman) Crazy way number one: bouncing.

(man) Look at yourself in a mirror. You probably do this often enough, but do it again. What do you see? It's yourself, of course. Have you ever wondered why this happens? You may have heard that light travels like a wave, like these water ripples that travel out. If they hit a wall, they bounce right back. The same thing happens with the mirror. Light from your cell phone hits your face and bounces off of it. This is called reflection. A lot of that reflected light bounces off the mirror and into your eyes.

(man) Crazy way number two: bending Let's look at this pencil underwater.

Why does the part of the pencil that's in the water look bent? For you to see the pencil, light needs to reflect off of it. But it also must travel through different materials before it hits your eye. Light takes the fastest path to get between two points. That might not always be a straight line. Light travels slower in water than in air, so the fastest path is bent. It's like traveling from your house. The shortest path isn't always the fastest. You could travel the shorter route, along slower, smaller roads, or go farther and take the fast highway.

(man) Light does that too.

This bending is called refraction.

(woman) So, the pencil looked bent, and your friend looked squiggly in the pool.

(man) Crazy way number three: mixing

(woman) Let's look at water waves, which behave similarly to light waves. I made these waves by dropping two droplets some distance apart. Each droplet forms waves going out that meet and mix with each other.

This mixing is called interference. At some meeting points, the mixed wave is bigger than the original waves. At other meeting points, the waves cancel each other out.

(man) We can see the same thing happening with light waves when I shine light through two tiny slits with a small separation. Like with water, some parts of light meet where the mixed light is brighter and other parts where the mixed light is darker. How does this concern invisibility cloaks and butterflies?

(woman) Let's bring these ways to mess with light together. How could this cloak make someone invisible?

(man) This camera is your eye, and this green fabric is the cloak we want to make invisible. You're probably thinking, "Make the cloak itself invisible so light would pass through it." If we did that, light would still bounce off what the cloak covers, pass through the cloak and reach your eyes. That's no good.

(woman) The cloak needs to take light from the opposite side of the camera and channel it through the fabric unchanged. No light would bounce off the cloak or what's covered inside. You'd just see what's on the opposite side of the cloak, like camouflage.

(man) Other sides of the cloak would channel light to its opposite side. No light would reach what's inside the cloak, and light hitting the cloak wouldn't be reflected into the camera lens. There'd be nothing to see.

(woman) All of this depends on channeling light, which is not easy. But nature can help. I'm with morpho butterflies at the Boston Museum of Science. Their wings are super shiny and bright blue. The front of their wings are made of alternating layers of air and chitin. Lobster shells are made of chitin. These alternating layers exploit how light bounces, bends, and mixes to channel every color of light except blue all the way to the back of the wing. The blues don't get channeled through, so you see blue on the surface but no other color. If you could take the color-channeling structures of a butterfly wing and weave them together into a fabric, you'd have cloak that would be invisible to every color of light except blue.

(man) In a world with no blue light, no blue could ever be reflected. So our cloak would work great! But if someone shined blue light on you and your cloak, you'd look silly. To work in our world, your cloak would have to channel all colors, including blue. How do you invent a true photonic invisibility cloak? Any photonic cloak should be possible, right? Because you can mess with how light bounces, bends, and mixes. I'm going to invent a photonic cloak first. I already have. You know, that's just a green cloak. Oh. Accessibility provided by the U.S. Department of Education.

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It's not just movie magic, discover the science behind invisibility cloaks. Two MIT scientists explain the physics of light and how an invisible cloak could theoretically work. Part of the "Science Out Loud" series.

Media Details

Runtime: 6 minutes

Science Out Loud
Episode 1
4 minutes
Grade Level: 9 - 12
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Episode 2
6 minutes
Grade Level: 10 - 12
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Episode 3
5 minutes
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Episode 4
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Episode 5
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Episode 6
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Episode 7
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Episode 8
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Episode 10
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