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- You might note that if you've got

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a positive charge sitting out in space

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it's gonna create an Electric field,

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and that Electric field is
gonna point radially outward

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away from the positive charge.

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And you might note that if
you've got a current in a wire,

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that current is gonna
create a Magnetic field,

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and that Magnetic field is
gonna loop around that wire

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it's gonna look something like this.

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But people started to realize

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there is another way to
create a Magnetic field

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besides having a current,

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and there's another way to
create an Electric field

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besides having charges.

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Turns out if you have a
changing Electric field

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in some region of space,

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even if that region has no current in it,

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by the fact that it has a
change in Electric field

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that's going to induce or
create a Magnetic field.

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So here was a way to
create a Magnetic field

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that didn't even required a current,

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it just required a change in
Electric field in that region.

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And people also started to realize

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if you've got a changing Magnetic field,

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that is gonna create a
changing Electric field.

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And a very clever Scottish physicist

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named James Clerk Maxwell realized,

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hold on, if this is true,

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if a changing Electric field

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can create a changing Magnetic field,

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and a changing Magnetic
field could in turn

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create a changing Electric field,

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which could in turn create
a changing Magnetic field,

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then you could set up some
sort of chain reaction here

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that can propagate like a wave,

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and you would get a wave

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of Electric and Magnetic fields

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that can travel outward and have

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sort of an independent existence now

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from whatever parent current

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or parent charge created them.

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So how would you create a
changing Electric field?

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You could take this charge

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and sort of wiggle it upside do--

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Up and down.

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Sounds like an antenna,

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that's what you could do, an antenna,

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that would create a
changing Electric field,

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and then a changing Magnetic field,

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and this could propagate outward.

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And similarly you could have a current,

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how could you create a
changing Magnetic field?

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Just have this current oscillate
back and forth, changing,

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that would create a
changing Magnetic field

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which would create a
changing Electric field,

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then it could propagate outward

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and now this wave could have

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its own independent existence

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and keep moving forward at some speed.

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And we call these
Electromagnetic waves because

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"Electro", they have Electric fields,

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and "magnetic" because
they have Magnetic fields,

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and waves because they propagate outward

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according to wave equations and

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similar to all the other
waves that we've seen.

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There is one difference,

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these waves don't actually need a medium,

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they can travel through straight vacuum,

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you don't need actual particles
of some stuff in here,

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these can move through the vacuum,

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which is a little weird

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because the only thing that's waving here,

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the only thing that's oscillating

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is the value of the Electric
and Magnetic fields.

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And so these are Electromagnetic waves,

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this is a pretty horrible drawing of it,

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it looks a little bit more like this.

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These yellow lines represent
the Magnetic fields,

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these are directed up and down,

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they point up and down,

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they don't stick out up and down,

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I just had to draw it that way

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but what I'm trying say is that

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this point right here,

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it's not that there's a
Magnetic field that sticks up,

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it points up,

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and I'll just draw with a bigger vector

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to show that at this point right here

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along with this axis,

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this axis could be, say the x direction,

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so if this is the x direction,

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and at this position in space

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there would be a relatively
large Magnetic field

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it would point up.

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And these red lines
represent the Electric field,

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so at this point in space

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there'd be a relatively
large Electric field,

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and the Electric field is at

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a right angle to the Magnetic field,

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so these are perpendicular,

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that's how this process has to happen.

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It's gonna create an
Electric and Magnetic field

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that are perpendicular to each other,

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so the Electric field sticks out,

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this is pointing, think of it

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as out of your computer
screen or your screen,

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or whatever you're watching this on.

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And these Electric fields back
here point into the screen.

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And so this is in 3D,
this is happening in 3D,

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Electric field oscillating
into and out of your screen,

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Magnetic field oscillating up and down,

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and the whole wave traveling to the right.

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So this whole thing happens,

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all three of these
directions are right angles,

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so if I imagine a
three-dimensional axes here,

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so x, y, z,

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this would be the direction
of the speed of this

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or the velocity of this wave,

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it doesn't have to point in this direction

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but whatever direction it's pointing

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you'd have velocity in one direction,

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you'd have Magnetic field
in the other direction,

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and you'd have Electric field

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in one more perpendicular direction,

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all three of these are
perpendicular to each other.

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So the Magnetic field is perpendicular

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to the Electric field,

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the Electric field is perpendicular
to the Magnetic field,

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then both the Magnetic
and the Electric fields

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are perpendicular to the direction

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that the Electromagnetic
wave is traveling.

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And the speed at which these waves travel

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is the speed of light, c,

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and by c I mean three
times 10 to the eight

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meters per second,

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because light is just
and Electromagnetic wave,

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light is a special example,
one particular example

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of Electromagnetic waves,

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but it is only one example,

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these waves can have any wavelength.

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Look, from here to here,
this, since this is in space,

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the graph of the wave and
along a special direction,

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this would represent the wavelength.

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These waves could have any wavelength,

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any particular wavelength,

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and any particular frequency.

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They frequency would be the rate

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at which these change.

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So if you'd watch this in time,

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this point in space would
have a Magnetic field

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that would point up,
then it would point down,

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then it would point up,
then it would point down,

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and the rate at which that's happening,

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the number of times per
second would be the frequency.

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These waves could have any frequency,

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but for one special region,

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the region is the visible spectrum.

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So we call the regional frequencies

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and wavelengths that
Electromagnetic waves can have

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the Electromagnetic Spectrum,

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and there is a lot to learn

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about the Electromagnetic Spectrum.

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Let me just show you really quick.

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If we pretend we're graphing
here, along this line,

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higher frequency.

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So let's say in that direction

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this is pointing higher
frequencies to the right,

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in other words higher Hertz

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since that's what we measure frequency in.

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But if that's higher frequency,

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because of this formula,

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speed of a wave is lambda
times frequency wavelength,

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times frequency.

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If the frequency increases the
wavelength has to decrease,

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so in this right way direction

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this would be smaller wavelength,

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and wavelength is measured in meters

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but when you're talking about light,

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since you have such small wavelengths

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you often talk about nanometers.

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So, where would the visible spectrum be?

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The visible spectrum would just be

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this little smudge here in our range,

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this would be the...

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This whole thing is called
the Electromagnetic Spectrum,

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so the Electromagnetic Spectrum,

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and the visible spectrum
is just right here.

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This would be the visible spectrum,

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so this is visible,

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between red and violet.

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And red is, if I asked you
what frequency that is,

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that would be around four times
10 to the fourteenth Hertz,

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and violet would be around

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7.5 times 10 to the fourteenth Hertz.

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Or if you wanted to talk about wavelength,

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red would be around 750 nanometers,

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and violet would be around 400 nanometers,

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and by nano we mean 10
to the negative ninth.

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Okay, but this is just a small region,

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you could have higher
frequencies than violet,

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and what do we call those?

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We call those ultraviolet,

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and we know these are bad for us,

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and they're bad for us because
as we go to the right here,

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higher frequencies,
turns out this also means

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00:08:31,446 --> 00:08:33,966
more energetic light.

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So if you were to talk about the photons

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that make up this light,

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if you're talking about quantum mechanic

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those photons as you go forward,

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higher up into the frequencies,

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have more energy,

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that means that they're more dangerous,

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and they're more dangerous because

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if they have more energy
they can add that energy

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00:08:52,033 --> 00:08:54,866
to your cells and break
them apart and cause damage,

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00:08:54,866 --> 00:08:56,600
and so this causes problems

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00:08:56,600 --> 00:08:57,866
as you go further up this way,

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so we put on sunscreen

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00:08:59,333 --> 00:09:02,100
to protect ourselves
from ultraviolet light.

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00:09:02,100 --> 00:09:04,633
But this isn't the highest frequency,

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00:09:04,633 --> 00:09:05,933
you can have much higher frequencies,

223
00:09:05,933 --> 00:09:07,933
you can have X-Rays,

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00:09:07,933 --> 00:09:09,833
X-Rays are even higher frequency,

225
00:09:09,833 --> 00:09:11,833
even more dangerous,

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00:09:11,833 --> 00:09:14,367
which is why X-Ray technicians,

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00:09:14,367 --> 00:09:15,333
you go in there and they're like,

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00:09:15,333 --> 00:09:16,466
"hey, stand right here,

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00:09:16,466 --> 00:09:17,633
"I'm going to get your X-Ray

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00:09:17,633 --> 00:09:19,966
"while I hide behind this wall",

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00:09:19,966 --> 00:09:21,200
because if they had to sit there

232
00:09:21,200 --> 00:09:23,600
and take that X-Ray every single day

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00:09:23,600 --> 00:09:26,200
it would really be bad for them.

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00:09:26,200 --> 00:09:28,033
But this isn't even the highest,

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00:09:28,033 --> 00:09:30,967
you've got things called Gamma Rays,

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00:09:30,967 --> 00:09:35,700
and Gamma Rays are
unbelievably energetic light,

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00:09:35,700 --> 00:09:36,933
and these are very dangerous.

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00:09:36,933 --> 00:09:39,400
These happens, these come from space

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00:09:39,400 --> 00:09:42,400
or from some sort of nuclear reaction.

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00:09:42,400 --> 00:09:45,300
These are extremely dangerous, Gamma Rays,

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00:09:45,300 --> 00:09:47,866
that's why you always
hear them in comic books,

242
00:09:47,866 --> 00:09:51,266
people wanna blame like
why some hero was created,

243
00:09:51,266 --> 00:09:53,033
they say, "oh, that was a Gamma Ray",

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00:09:53,033 --> 00:09:54,933
'cause it just sounds really powerful.

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00:09:54,933 --> 00:09:56,633
It won't turn people into superheroes

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00:09:56,633 --> 00:09:58,700
but it's very energetic.

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00:09:58,700 --> 00:10:02,667
Then down here you've got infrared,

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00:10:02,667 --> 00:10:06,000
so this would be below red.

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00:10:06,533 --> 00:10:07,966
You couldn't see these either,

250
00:10:07,966 --> 00:10:09,000
could be either infrared,

251
00:10:09,000 --> 00:10:12,266
and then below that you've got microwaves,

252
00:10:12,266 --> 00:10:15,266
and microwaves, in this region

253
00:10:15,266 --> 00:10:16,733
there's a lot of useful stuff,

254
00:10:16,733 --> 00:10:20,800
your cellphones signals,
they're in the microwave region,

255
00:10:20,800 --> 00:10:23,066
TV signals that are sent through the air

256
00:10:23,066 --> 00:10:24,400
in the microwave region,

257
00:10:24,400 --> 00:10:26,133
this is used for a lot of cases.

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00:10:26,133 --> 00:10:29,433
And then below this
you've got radio waves,

259
00:10:29,433 --> 00:10:33,233
even though technically
speaking FM radio waves

260
00:10:33,233 --> 00:10:35,400
are more in the microwave region,

261
00:10:35,400 --> 00:10:36,866
these are more like AM,

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00:10:36,866 --> 00:10:40,167
so when people started first making radio

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00:10:40,167 --> 00:10:42,766
they used frequencies down here,

264
00:10:42,766 --> 00:10:45,933
and that was the AM frequency range.

265
00:10:45,933 --> 00:10:48,100
So this is the Electromagnetic Spectrum,

266
00:10:48,100 --> 00:10:51,133
the visible range is a small region of it,

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we can't see anything outside of that,

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but these are extremely useful

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in a lot of different cases,

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and dangerous, you need to know about

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the Electromagnetic Spectrum.