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.