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  • What I want to do in this video is give ourselves

  • a basic introduction to the phenomenon of light.

  • And light is, at least to me, mysterious.

  • Because on one level it really defines our reality.

  • It's maybe the most defining characteristic of our reality.

  • Everything we see, how we perceive reality,

  • is based on light bouncing off of objects

  • or bending around objects or diffracting around objects,

  • and then being sensed by our eyes,

  • and then sending signals into our brain that

  • create models of the world we see around us.

  • So it really is, almost, the defining characteristic

  • of our reality.

  • But at the same time, when you really go down to experiment

  • and observe with light, it starts

  • to have a bunch of mysterious properties.

  • And to a large degree it is not fully understood yet.

  • And probably the most amazing thing about light--

  • well, actually there's tons of amazing things about light--

  • but one of the mysterious things is when you really get down

  • to it-- and this is actually not just true of light,

  • this is actually true of almost anything

  • once you get onto a small enough quantum mechanical level--

  • light behaves as both a wave and a particle.

  • And this is probably not that intuitive to you,

  • because it's not that intuitive to me.

  • In my life, I'm used to certain things behaving as waves,

  • like sound waves or the waves of an ocean.

  • And I'm used to certain things behaving like particles,

  • like basketballs or-- I don't know-- my coffee cup.

  • I'm not used to things behaving as both.

  • And it really depends on what experiment you run

  • and how you observe the light.

  • So when you observe it as a particle,

  • and this comes out of Einstein's work

  • with the photoelectric effect-- and I won't go into the details

  • here, maybe in a future video when

  • we start thinking about quantum mechanics--

  • you can view light as a train of particles moving

  • at the speed of light, which I'll talk about in a second.

  • We call these particles photons.

  • If you view light in other ways-- and you see it

  • even when you see light being refracted by a prism here--

  • it looks like it is a wave.

  • And it has the properties of a wave.

  • It has a frequency, and it has a wavelength.

  • And like other waves, the velocity of that wave

  • is the frequency times its wavelength.

  • Now even if you ignore this particle aspect of light,

  • if you just look at the wave aspect of the light,

  • it's still fascinating.

  • Because most waves require a medium to travel through.

  • So for example, if I think about how sound travels through air--

  • so let me draw a bunch of air particles.

  • I'll draw a sound wave traveling through the air particles.

  • What happens in a sound wave is you compress some of the air

  • particles and those compress the ones next to them.

  • And so you have points in the air that have higher,

  • I guess you could say, higher pressure

  • and points that have lower pressure,

  • and you could plot that.

  • So we have high pressure over here.

  • High pressure, low pressure, high pressure, low pressure.

  • And as these things bump into each other,

  • and this wave essentially travels to the right--

  • and if you were to plot that you would see this wave

  • form traveling to the right.

  • But this is all predicated, or this is all

  • based on, this energy traveling through a medium.

  • And I'm used to visualizing waves in that way.

  • But light needs no medium.

  • Light will actually travel fastest through nothing,

  • through a vacuum.

  • And it will travel at an unimaginably fast speed--

  • 3 times 10 to the eighth meters per second.

  • And just to give you a sense of this,

  • this is 300 million meters per second.

  • Or another way of thinking about it is it

  • would take light less than a seventh of a second

  • to travel around the earth.

  • Or it would travel around the earth more than seven times

  • in one second.

  • So unimaginably fast.

  • And not only is this just a super fast speed,

  • but once again it tells us that light

  • is something fundamental to our universe.

  • Because it's not just an unimaginable fast speed.

  • It is the fastest speed not just known to physics, but possible

  • in physics.

  • So once again something very unintuitive to us

  • in our everyday realm.

  • We always imagine that, OK, if something

  • is going at some speed, maybe if there was an ant riding on top

  • of that something and it was moving in the same direction,

  • it would be going even faster.

  • But nothing can go faster than the speed of light.

  • It's absolutely impossible based on our current understanding

  • of physics.

  • So it's not just a fast speed, it

  • is the fastest speed possible.

  • And this right here is an approximation.

  • It's actually 2.99 something something times 10

  • to the eighth meters per second.

  • But 3 times 10 to the eighth meters per second

  • is a pretty good approximation.

  • Now within the visible light spectrum-- and I'll

  • talk about what's beyond the visible light spectrum

  • in a second-- you're probably familiar with the colors.

  • Maybe you imagine them as the colors of the rainbow.

  • And rainbows really happen because the light

  • from the sun, the white light, is being refracted

  • by these little water particles.

  • And you can see that in a clearer way when

  • you see light being refracted by a prism right over here.

  • And the different wavelengths of light-- so white light

  • contains all of the visible wavelengths--

  • but the different wavelengths get refracted differently

  • by a prism.

  • So in this case the higher-frequency wavelengths,

  • the violet and the blue, get refracted more.

  • Its direction gets bent more than the low-frequency

  • wavelengths, than the reds and the oranges right over here.

  • And if you want to look at the wavelength of visible light,

  • it's between 400 nanometers and 700 nanometers.

  • And the higher the frequency, the higher the energy of that

  • light.

  • And that actually goes into when you

  • start talking about the quantum mechanics of it--

  • that the higher frequency means that each of these photons

  • have higher energy.

  • They have a better ability to give kinetic energy

  • to knock off electrons or whatever else they need to do.

  • So higher frequency-- let me write

  • that down-- higher frequency means higher energy.

  • Now I keep referring to this idea of the visible light.

  • And you might say, what is beyond visible light?

  • And what you'll find is that light is just

  • part of a much broader phenomenon,

  • and it's just the part that we happen to observe.

  • And if we want to broaden the discussion a little bit,

  • visible light is just really part

  • of the electromagnetic spectrum.

  • So light is really just electromagnetic radiation.

  • And everything that I told you about light just now--

  • it has a wave property and it has particle properties--

  • this is not just specific to visible light.

  • This is true of all of electromagnetic radiation.

  • So at very low frequencies or very long wavelengths--

  • we're talking about things like radio waves,

  • the things that allow a radio to reach your car;

  • the things that allow your cellphone

  • to communicate with cell towers; microwaves, the things that

  • start vibrating water molecules in your food

  • so that they heat up; infrared, which

  • is what our body releases, and that's

  • why you can detect people through walls

  • with infrared cameras; visible light; ultraviolet light,

  • the UV light coming from the sun that'll give you sunburn;

  • X-rays, the radiation that allows

  • us to see through the soft material and just visualize

  • the bones; gamma rays,

  • the super high energy that comes from quasars

  • and other certain types of physical phenomena-- these

  • are all examples of the exact same thing.

  • We just happen to perceive certain frequencies

  • of this as visible light.

  • And you might say, hey, Sal, how come we only

  • perceive certain frequencies of this?

  • How can we only see these frequencies?

  • Literally we can see those frequencies

  • with our unaided eye.

  • And the reason, or at least my best guess

  • of the reason of that, is that's the frequency where

  • the sun dumps out a lot of electromagnetic radiation.

  • So it's inundating the Earth.

  • And if, as a species, you wanted to observe things

  • based on reflected electromagnetic energy,

  • it is most useful to be able to perceive the things where there

  • is the most electromagnetic radiation.

  • So it is possible that in other realities or other planets

  • there are species that perceive more

  • in the ultraviolet range or the infrared range.

  • And even on Earth, there are some

  • that perform better at either end of the range.

  • But we see really well in the part of the spectrum

  • where the sun just happens to dump a lot of radiation on us.

  • Now I'll leave you there.

  • I think that's a pretty good overview of light.

  • And if any of this stuff seems kind

  • of unintuitive or daunting, or really

  • on some level confusing-- this wave-particle duality,

  • this idea of a transfer of energy through nothing--

  • and it seems unintuitive, don't worry.

  • It seems unintuitive even for the best of physicists.

  • So you're already at the leading edge of physics thinking.

What I want to do in this video is give ourselves

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B1 visible light wave frequency visible electromagnetic speed

Introduction to Light

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    Kevin Tan posted on 2014/08/16
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