Introduction to light | Electronic structure of atoms | Chemistry | Khan Academy

Summary

This video offers an introduction to the phenomenon of light, highlighting its dual nature as both a wave and a particle. It explains how light shapes our perception of reality, discussing its behavior, speed, and role in the electromagnetic spectrum. The video touches on quantum mechanics, the photoelectric effect, and how light interacts with objects, creating models in our brain. It also explores why humans perceive certain frequencies as visible light, emphasizing the importance of light in physics and the mystery behind its properties.

Takeaways

• 🌟 Light is a fundamental phenomenon that defines how we perceive reality by interacting with objects and being sensed by our eyes.
• 💡 Light behaves both as a wave and as particles called photons, depending on the experiment and observation method.
• 🌊 Light as a wave has properties like frequency, wavelength, and travels at the fastest possible speed in physics: 3 x 10^8 meters per second (speed of light).
• 🌍 Light can travel through a vacuum without needing a medium, unlike sound, which requires air or another material.
• 🌈 Visible light is only a small part of the electromagnetic spectrum, with wavelengths ranging from 400 to 700 nanometers.
• 🔥 Higher frequency light (like violet and blue) has higher energy, while lower frequency light (like red) has lower energy.
• 📻 The electromagnetic spectrum includes a wide range of radiation beyond visible light, such as radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
• 🌞 We perceive visible light because the sun emits a significant amount of electromagnetic radiation in this range, making it useful for species on Earth to detect.
• 🔍 Other species or life forms might perceive different parts of the electromagnetic spectrum, like ultraviolet or infrared, based on their environment.
• 🤔 The wave-particle duality of light and its ability to travel through nothingness are counterintuitive, even for advanced physicists, making light a mysterious and fascinating phenomenon.

Q & A

What is the basic definition of light according to the script?

Light is a fundamental characteristic of reality, as it defines how we perceive the world through its interaction with objects, bouncing or bending, and being sensed by our eyes.

Why is light considered mysterious?

Light is considered mysterious because, despite defining our perception of reality, it exhibits properties that are not fully understood. Most notably, it behaves as both a wave and a particle, which is counterintuitive.

What is wave-particle duality?

Wave-particle duality refers to the phenomenon where light behaves as both a wave and a particle, depending on how it is observed. This dual behavior is seen in other quantum mechanical phenomena as well.

How does light behave as a particle?

As a particle, light can be viewed as a train of photons moving at the speed of light, as demonstrated by Einstein's work on the photoelectric effect.

How does light behave as a wave?

As a wave, light has properties such as frequency and wavelength. When light is refracted, such as through a prism, it behaves like a wave with different wavelengths bending at different angles.

What is unique about light compared to other waves like sound?

Unlike sound waves, which require a medium to travel through, light can travel through a vacuum and does not need a medium. It travels fastest in a vacuum at approximately 3 x 10^8 meters per second.

Why is the speed of light significant in physics?

The speed of light is significant because it is not just fast, but the fastest speed possible according to our current understanding of physics. Nothing can exceed this speed.

What is the visible light spectrum?

The visible light spectrum is the range of electromagnetic radiation that humans can perceive, with wavelengths between 400 and 700 nanometers. It includes colors like red, blue, and violet.

What is the relationship between frequency and energy in light?

Higher frequency light has higher energy. For instance, violet light has a higher frequency and energy than red light, and this is relevant when discussing the quantum mechanics of photons.

How does the electromagnetic spectrum extend beyond visible light?

The electromagnetic spectrum includes not only visible light but also other types of electromagnetic radiation like radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays, all of which share wave-particle duality.

Outlines

00:00

💡 Introduction to the Mystery of Light

The speaker introduces the concept of light, highlighting its mysterious nature and how it defines our reality. Light is responsible for how we perceive the world around us, as it bounces, bends, or diffracts off objects and is processed by our brain. While light has well-understood properties, such as being essential to vision, it also exhibits puzzling quantum behavior. Light can act both as a wave and as a particle depending on how it's observed, which is non-intuitive because in everyday life, we're used to objects behaving exclusively as either waves or particles. The speaker references Einstein's work on the photoelectric effect, introducing photons as particles of light, while also explaining light's wave-like properties like frequency and wavelength.

05:02

🌈 The Speed and Nature of Light

The discussion shifts to the speed of light, which is the fastest known and possible speed in the universe, clocking in at approximately 300 million meters per second. Light's speed, unaffected by a medium, travels fastest through a vacuum. The speaker uses various analogies, like sound waves needing a medium, to highlight how unique light is. Light’s extraordinary speed further underlines its fundamental role in the universe, with no object or entity capable of surpassing it, according to current physics. This section wraps up with a reminder that the speed of light not only represents an exceptionally fast speed but also the ultimate limit in the physical world.

Keywords

💡Light

Light is the central topic of the video and is described as a mysterious phenomenon that defines how we perceive reality. It behaves both as a wave and a particle, depending on how it is observed. The video highlights its role in shaping our understanding of the world through its interaction with objects, whether by bouncing, bending, or diffracting.

💡Wave-particle duality

Wave-particle duality refers to light's ability to behave both as a wave and a particle, a concept that is counterintuitive but crucial in modern physics. The video emphasizes this duality by explaining that light can exhibit wave-like properties in phenomena such as refraction and particle-like properties when viewed as photons, as seen in the photoelectric effect.

💡Photon

A photon is a particle of light, introduced in the context of Einstein's work with the photoelectric effect. The video explains that photons are the particle form of light, and they travel at the speed of light. This particle aspect of light helps in understanding how light transfers energy and interacts with matter at a quantum level.

💡Electromagnetic spectrum

The electromagnetic spectrum includes all types of electromagnetic radiation, of which visible light is only a small part. The video explains that light exists within a broad range of frequencies, from low-frequency radio waves to high-frequency gamma rays, and each type of radiation shares the same wave-particle duality properties.

💡Frequency

Frequency is the number of waves that pass a point in a given amount of time. In the video, higher frequencies are associated with more energetic light, such as violet light, which gets refracted more by a prism than lower-frequency light like red. Higher frequency also correlates to higher energy in the electromagnetic spectrum.

💡Wavelength

Wavelength is the distance between two consecutive peaks of a wave. In the context of light, the video explains how different wavelengths of visible light are responsible for the colors we perceive. For example, light with shorter wavelengths like violet has more energy than light with longer wavelengths like red.

💡Speed of light

The speed of light is a constant at approximately 3 x 10^8 meters per second. The video highlights how light travels faster than anything else in the universe, making it a fundamental concept in physics. It also explains that light can travel through a vacuum without the need for a medium, unlike sound waves.

💡Refraction

Refraction is the bending of light as it passes through a medium, such as a prism or water particles in the atmosphere. The video uses the example of white light being refracted through a prism, splitting into different colors because the varying wavelengths of light are bent by different amounts.

💡Quantum mechanics

Quantum mechanics is the branch of physics that explains the behavior of particles on a very small scale, including light's wave-particle duality. The video touches on this by mentioning that light's behavior becomes mysterious and less intuitive when observed at the quantum level, such as in the photoelectric effect and photon interactions.

💡Electromagnetic radiation

Electromagnetic radiation is the broader concept that includes all forms of light, from radio waves to gamma rays. The video explains that visible light is just one part of this spectrum, and all electromagnetic radiation shares similar properties, including the ability to behave as both waves and particles.

Highlights

Light is a mysterious phenomenon that defines our reality by bouncing off objects, bending, or diffracting and is then sensed by our eyes.

Light behaves as both a wave and a particle, a duality that is not fully intuitive or understood.

In Einstein's work on the photoelectric effect, light can be seen as particles called photons moving at the speed of light.

Light as a wave has properties like frequency and wavelength, with its velocity being the product of both.

Unlike sound waves, which require a medium, light travels fastest through a vacuum and needs no medium.

The speed of light is unimaginably fast at approximately 300 million meters per second.

Light travels around the Earth more than seven times in one second.

Nothing can travel faster than the speed of light, making it the fastest possible speed in physics.

Different wavelengths of visible light, when refracted, show as colors of the rainbow, from violet to red.

Visible light has wavelengths between 400 and 700 nanometers, with higher frequencies corresponding to higher energy photons.

Visible light is part of a broader electromagnetic spectrum, which includes radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.

The human eye perceives only a small part of the electromagnetic spectrum, where the sun emits most of its radiation.

Other species might see more in the ultraviolet or infrared ranges, depending on their environment.

The behavior of light as both a wave and a particle, and its ability to travel through a vacuum, challenges our everyday understanding of physics.

Even for expert physicists, the wave-particle duality and properties of light remain unintuitive and mysterious.

Transcripts

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.