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What if you could only see one color?

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Imagine, for instance,

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that you could only see things that were red

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and that everything else

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was completely invisible to you.

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As it turns out,

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that's how you live your life all the time

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because your eyes can only see

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a minuscule part of the full spectrum of light.

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Different kinds of light are all around you everyday

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but are invisible to the human eye,

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from the radio waves that carry your favorite songs,

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to the x-rays doctors use to see inside of you,

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to the microwaves that heat up your food.

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In order to understand

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how these can all be light,

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we'll need to know a thing or two

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about what light is.

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Light is electromagnetic radiation

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that acts like both a wave and a particle.

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Light waves are kind of like waves on the ocean.

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There are big waves and small waves,

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waves that crash on the shore

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one right after the other,

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and waves that only roll in every so often.

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The size of a wave is called its wavelength,

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and how often it comes by

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is called its frequency.

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Imagine being a boat in that ocean,

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bobbing up and down as the waves go by.

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If the waves that day have long wavelengths,

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they'll make you bob only so often,

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or at a low frequency.

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If the waves, instead, have short wavelengths,

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they'll be close together,

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and you'll bob up and down much more often,

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at a high frequency.

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Different kinds of light are all waves,

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they just have different wavelengths and frequencies.

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If you know the wavelength or frequency

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of a wave of light,

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you can also figure out its energy.

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Long wavelengths have low energies,

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while short wavelengths have high energies.

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It's easy to remember

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if you think about being in that boat.

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If you were out sailing on a day

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with short, choppy waves,

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you'd probably be pretty high energy yourself,

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running around to keep things from falling over.

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But on a long wavelength sea,

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you'd be rolling along, relaxed,

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low energy.

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The energy of light tells us

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how it will interact with matter,

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for example, the cells of our eyes.

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When we see, it's because the energy of light

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stimulates a receptor in our eye

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called the retina.

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Our retina are only sensitive to light

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with a very small range in energy,

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and so we call that range of light visible light.

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Inside our retina are special receptors

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called rods and cones.

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The rods measure brightness,

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so we know how much light there is.

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The cones are in charge of what color of light we see

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because different cones are sensitive

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to different energies of light.

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Some cones are more excited by light

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that is long wavelength and low energy,

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and other cones are more excited

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by short wavelength, high-energy light.

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When light hits our eye,

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the relative amount of energy each cone measures

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signals our brain to perceive colors.

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The rainbow we perceive

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is actually visible light in order of its energy.

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At one side of the rainbow

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is low-energy light we see as red,

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and at the other side is high-energy light

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we see as blue.

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If light shines on us

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that has an energy our retina can't measure,

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we won't be able to see it.

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Light that is too short wavelength or high energy

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gets absorbed by the eye's surface

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before it can even get to the retina,

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and light that is too long wavelength

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doesn't have enough energy

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to stimulate our retina at all.

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The only thing that makes one kind of light

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different from another is its wavelength.

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Radio waves have long wavelengths,

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while x-rays have short wavelengths.

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And visible light, the kind you can actually see,

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is somewhere in between.

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Even though our eyes can't detect light

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outside of the visible range,

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we can build special detectors

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that are stimulated

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by these other wavelengths of light,

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kind of like digital eyes.

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With these devices,

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we can measure the light that is there,

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even though we can't see it ourselves.

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So, take a step back and think about

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all of this for a moment.

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Even though they seem different,

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the warmth you feel from a crackling fire

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is the same as the sun shining on you

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on a beautiful day,

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the same as ultraviolet light

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you put on sunscreen to protect yourself from,

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the same thing as your TV,

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your radio,

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and your microwave.

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Now, those examples are all things here on Earth,

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things you experience in your everyday life,

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but here's something even more amazing.

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Our universe gives off the full spectrum of light, too.

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When you think of the night sky,

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you probably think of being able

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to see the stars shining with your own eyes,

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but that's just visible light,

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which you now know is only a tiny part

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of the full spectrum.

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If we had to draw the universe

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and could only use visible light,

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it would be like having only one crayon --

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pretty sad.

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To see the universe in its full spectrum,

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we need to have the right eyes,

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and that means using special telescopes

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that can help us see beyond visible light.

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You've probably heard of the Hubble Space Telescope

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and seen its beautiful pictures

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taken in visible and ultraviolet light.

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But you might not know

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that there are 20 space telescopes in orbit,

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missions that can each see part

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of the full spectrum of light.

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With telescopes acting as our virtual eyes,

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both in space and here on Earth,

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we can see some amazing things.

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And the coolest thing of all,

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no matter the wavelength or energy,

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the light that we see out in the distant universe

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is the same thing as the light

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that we can experience and study here on Earth.

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So, since we know the physics

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of how x-ray,

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ultraviolet light,

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or microwaves work here,

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we can study the light of a distant star or galaxy

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and know what kinds of things

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are happening there too.

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So, as you go about your daily life,

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think beyond what your eyes can and can't see.

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Knowing just a little bit about the natural world

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can help you perceive the full spectrum

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around you all the time.