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What I want to do in this video is give ourselves

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a basic introduction to the phenomenon of light.

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And light is, atleast to me, it is mysterious

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because on one level it really defines our reality

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it may be the most defining caracteristic of our reality

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everything we see, how we percieve reality is based

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on light bouncing off of objects, or bending around objects or difffracting around objects.

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and then being sensed by our eyes and then

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sending signals into our brain that create models of the world we see around us

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so it really is almost a defining

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characteristic of our reality, but at the same time,

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when you really go down to experiment and observe with light,

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its starts to have a bunch of mysterious

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properties and to a large degree it is not fully understood yet.

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And probably the most amazing thing about light

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...well, actually there is tons of thing amazing about light....

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but one of the mysterious things is

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when you really get down to it, and this is actually not just true of light,

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this is actually true of almost anything once you

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get onto a small enough quantum mechanical level,

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but light behaves as both a wave AND a particle.

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And this is, probably not that intuitive to you, because its

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not that intuitive to me!

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In my life I'm used to certain things

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behaving as a waves, like sound waves,

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or the waves of an ocean.

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And I'm used to certain things behaving like particles.

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Like... Basket balls, or... I don't know...

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Or... My coffee cup. I'm not used to things

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behaving as both!

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And it really depends on what experiment

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you run and how you observe the light.

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So, when you observe it as a particle,

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And this comes out of Einstein's work with the photo-electric effect.

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And I won't go into the details here, maybe in a future video

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when we start thinking about quantum mechanics.

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You can view light as a train of particles moving at the speed of light.

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Which I'll talk about in a second...

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We call these particles PHOTONS.

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If you view light in other ways

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and you see it even when you see light beam

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refracted by a prism here, it looks like it is a wave.

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And it has the properties of a wave.

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It has a frequency and it has a wavelength.

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And like other waves, the velocity, of that wave, is the frequency times its wave length [v = f * lambda]

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

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If you just look at the wave aspect of the light it's still fascinating because

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most waves require a medium to travel through.

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So for example: If I think about how

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sound travels through air.

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So let me draw a bunch of air particles.

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Air particles right here

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

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What happens in a sound wave is:

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You compress some of the air particles

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and those compress the ones next to them.

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And so you have points in the air that have higher, I guess you could say, higher pressure

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and points that have lower pressure,

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and you can plot that.

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So we have high pressure over here. High pressure, low pressure, high pressure, low pressure...

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And as these things bump into each other and this wave essentially travels

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to the right, and if you were to plot that

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you would see this wave form traveling to the right.

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But this is all predicated, or this is all based on this energy

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traveling through a medium and I'm used to visualizing waves in that way but light NEEDS NO MEDIUM!

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Light needs no medium

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Light will actually travel fastest through nothing, through a vacuum.

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And it will travel at an unimaginably fast speed: 3*10^8 m/s.

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Just to give you a sense of this:

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This is 300 million meters per second. Or another way

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of thinking about it is: It would take light less than a

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seventh of a second to travel around the earth, or

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it would travel around the earth more then seven times in a second.

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So unimaglably fast. And not only is this just a super fast rate,

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or a super fast speed but once again

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it tells us that light is something fundamental to our universe,

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because it's not just an unimaginable fast speed, it is THE FASTEST speed,

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not just known to physics, but POSSIBLE in physics.

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So once again something very unintuitive to us in our everyday realm.

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We always imagine that: "OK, if something is going at some speed,

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maybe if there is a ant riding on top of that something

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moving in the same direction it would be going even faster.

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But nothing can go faster that the SPEED OF LIGHT.

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It's absolutely impossible based on our current understanding of physics.

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So its not just a fast speed it is THE FASTEST SPEED. It is the fastest speed possible

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And this right here is an approximation,

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it's actually 2.99 something, something times 10 to the 8th meters per second...

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But 3 times 10 to the 8th is a pretty good approximation.

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Now within the visible light spectrum and I'll talk about

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whats beyond the visible light spectruum in a second.

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You are probablly familiar with

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colors and maybe you imagine it as a colors of rainbow,

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and rainbow really happen because the light from the sun,

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the white light, is being refracted by these little water particles.

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and you can see that in a clearer way when you see

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light being refracted by a prism right over here.

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and the different wavelengths of light, so white light

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contains all of the visible wavelengths.

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But the different wavelengths get refracted differently by a prism.

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So in this case the higher frequenty wavelengths,

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the violet and the blue, get refracted more, its get bent,

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it's direction gets bent more than the low frequency wavelengths,

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than the reds and the oranges right over here

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

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visible light, its between 400 nanometeres and 700 nanometers.

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And the higher the frequency, the higher the energy of that light.

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And that actually goes into, when you start talking

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about the quantuum mechanics of it.

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That the higher frequency means that each of these photons have higher energy.

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They have a better ability to give kinetic energy to knock off electrons,

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or whatever else they need to do.

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So higher frequency, let me write that down:

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[HIGHER FREQUNCY means HIGHER ENERGY]

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

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And you might say: "What is beyond visible light?"

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And what you'll find is that light is just a part

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of a much broader phenomenom that is just a part

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that we happen to observe.

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And if we want to broaden the discussion a little bit.

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Visible light is just, really, part of the electromagnetic spectrum

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So light is really just electromagentic radiation.

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[Electromagnetic radiation]

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And everything that I told you about light just now,

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it has a wave property and it has particle properties,

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This is not just specific to visible light.

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This is true of all of electromagnetic radiation.

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So at very low frequencies, or very long wavelengths,

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we're talking about things like radio waves,

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the things that allow radio to reach your car,

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the things that allow your cell phone to

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comunicate with cell towers.

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Microwaves, the thing that start vibrating water molecules

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in your food so that they heat up.

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Infrared, wich is what our body realeses and thats

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way you can detect people through walls with infrared cameras.

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Visible light... Ultraviolet light, the UV light coming

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from the sun that'll give you sunburn.

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X-rays! The radiation that allows us to see through

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the soft material and just visualize the bones.

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Gamma rays! The super high energy that comes from quasars

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and other certain types of physical phenomenons.

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These are all the examples of the exact same thing!

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We just happen to perceive certain frequencies of this as visible light!

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And you might say: "Hey, Sal, how come

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we only perceive certain frequancies of this?

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How come we only see these frequencies?"

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And I'll put these...

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Literally we can see those frequencies with our unaided eye.

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And the reason, or at least my best guess of the reason of that is:

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That's the frequency where the sun dumps out a lot of electromagnetic radiation.

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So its illumating the earth, and if

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as a species you wanted to observe things

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based on reflected energy, a reflected electromagnetic energy,

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it is most usefull to be able to perceive the things

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where there is the most electromagnetic radiation.

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So it is possible that in other realities or other plantes

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There are species that perceive more in the ultraviolet range,

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or infrared range.

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Or even on earth there are some

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that perform better at either end of the range.

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But we see really well in the part of the spectrum

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

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Now I'll leave you there, I think that's a pretty good overview of light.

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And if any of this stuff seems kind of unintuitive or

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daunting or really on some level confusing

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this wave-particle duality, this idea of

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of transfer of energy through nothing

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and it seems unintuitive: Don't worry!

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It seems unintuitive even for the best of physicists.

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