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The physics of invisibility cloaks

Video transcript

every morning you see your reflection in the mirror every summer your friend looks squiggly in the bottom of the pool but how can we use this to make a disability close which is all the crazy ways to mess with lights crazy way number one bouncing take a look at yourself in a mirror you probably do this often enough but do it again anyway what do you see it's yourself of course but have you ever wondered why this happens you may have heard that light travels like a wave sort of like these water waves the water ripples have high parts and low parts that travel out and if they hit a wall they bounce right back that's the same sort of thing going on with the mirror light coming from your cell phone is hitting your face and bouncing off of it this bouncing is called reflection a lot of that reflected light bounces off the mirror and goes into your eyes crazy way number two bending now let's look at this pencil underwater why does the part of the pencil that's in the water look bent compared to the part above it of course for you to see the pencil at all white needs to reflect off of it but it also has to travel through different materials before it hits your eye light generally takes the fastest path to get between two points and that might not always be a straight line light travels slower in water than in air so the fastest path is actually Bank it's just like when you travel from your house the shortest path isn't always the fastest like here you could travel the shorter route along slower smaller roads or you could go a little farther and take the fast highway light does that too this bending is called refraction that's why the pencil looked bent in the cup and your friend looks squiggly in the pool crazy way number three mixing let's look at water waves which actually behave pretty similarly to light waves I made these waves by dropping two droplets some distance apart each droplet forms a set of waves going out and these waves meet and mix with each other this mixing is called interference at some meeting points the mixed wave is bigger than each of the original two waves and other meeting points the two waves cancel each other out we can see the same thing happening with light waves in this experiment where I shine light through two tiny slits with a small separation just like with water there are some parts of light meeting where the mixed light is brighter and other parts where the mixed light is darker well what does this have to do with the invisibility cloaks and butterflies let's bring these crazy ways to mess with light together how could this cloak make someone invisible let's say this camera is your eye and this green fabric is the cloak we want to make invisible you're probably thinking let's make the cloak itself invisible so that light would pass through it but if we did that light would still be able to bounce off what the quote covers pass through the cloak and get to your eyes and that's no good what the cloak really needs to do is take light from the opposite side of the camera channel it through the fabric and out the front unchanged no light would bounce off the cloak or what it covers inside so you would just see what's on the opposite side of the cloak kind of like camouflage in every other side of the cloak which is channel like to its opposite side so none of the light would even get to what's inside the cloak and none of the light hitting the cloak would be reflected into the camera lens so there would be nothing to see all this depends on channeling light which is not easy but nature can help I'm here with some morpho butterflies at the Boston Museum of Science their wings are super shiny bright blue color the front of their wings are made of alternating layers of air and chitin and chitin is what lobster shells are made of these alternating layers take advantage of how light bounces bends and mixes to channel the reds yellows greens oranges basically every color of light except blue all the way to the back of the wing the Blues don't get channeled through so this is why 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 would have a cloak that would be invisible to every color of light except blue if we lived in a world with no blue light no blue could ever be reflected sir cloak would work great with someone shined a blue light on you and your cloak you would look kind of silly to work in our world your clerk would have to channel all colors including blue so how do you invent the truth for coming in disability Club well any botana cloak should be possible right because you can mess with how light bounces bends and mixes all right well I'm going to invent a photonic coil first I already you know that's just a green cloak [Music]