ShmoopTube

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[Dino and Coop singing]

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We all know that the sky is blue. [Girl holding a sign that says blue]

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And if you didn't know that, then…how.

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Like, how.

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Are you even a human…? [Guy holding a sign that says red face falls off to reveal a metal body]

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Anyway.

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But what if we asked: "Why is the sky blue?"

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Is there a deeper reason behind it?

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Is blue more poetic than, say…highlighter yellow? [The sky is colored yellow with a highlighter]

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Luckily, we won't need to spend five years getting a PhD in philosophy to figure out [People graduating]

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which color is the most poetic.

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It all comes down to different frequencies. [Girl is pulled off the stage with a vaudeville hook]

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As you might remember, light is a form of energy called electromagnetic radiation. [Coop pointing at a blackboard]

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And depending on the frequency of this electromagnetic radiation, we get different kinds of light. [Dino pointing at a blackboard]

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So waves with a slightly higher frequency than visible light are ultraviolet radiation… [UV radiation hitting guy holding a surf board on the beach]

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…and those with a slightly lower frequency are infrared radiation. [Picture from an infrared camera]

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In fact, if the frequency gets lower than infrared radiation, you can even get radio [A radio tower]

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waves.

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So different frequencies give us very different kinds of light. [Someone tuning a radio]

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And no, pouring green paint all over your radio won't help you pick up any Top 40 stations. [Guy throws green paint onto the radio]

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Sorry about that.

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The idea that a different frequency produces a different kind of light also applies when

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we look at the range of visible light. [The electromagnetic spectrum]

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You might remember that visible light has frequencies that range between 400 and 800

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tetrahertz.

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We can break that big 400 tetrahertz range down into smaller chunks that give us all

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of the colors of the rainbow: red between about 400 and 480 THz… [All the different color wavelengths are shown]

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…orange between about 480 and 510 THz…

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…yellow between about 510 and 525 THz…

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…green between about 525 and 610 THz…

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…blue between about 610 and 670 THz, and violet between about 670 and 800 THz.

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Okay, so not all the colors of the rainbow, we left out indigo. [A rainbow appears]

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So sue us.

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These different wavelengths have different effects on our eyes. [Wavelengths of light entering an eye]

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When light travels into a human eye, it hits the retina, which translates the light into

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a bunch of signals that the brain can understand. [Coop pointing at a blackboard]

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Different parts of the retina detect the frequency of the light…

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…and then the retina passes that info along to the optic nerve, so that it can fill the

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brain in on what color the eyes see. [Wavelength signals arrive at the brain]

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This same process works for any color, whether you're looking at a blue sky with its colors

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in the 610 to 670 THz range…

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…or a yellow sky, with its colors between 510 and 525 THz.

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But if you're seeing a yellow sky, you might want to find an underground bunker and hide…sounds [A fire breathing dragon appears]

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like the apocalypse is near. [Guy running away as he sees the dragon in the window]

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