Introduction to light Light and electromagnetic radiation
Introduction to light
- What I want to do in this video is give ourselves
- a basic introduction to the phenomenon of light.
- And light is, atleast to me, it is mysterious
- because on one level it really defines our reality
- it may be the most defining caracteristic of our reality
- everything we see, how we percieve reality is based
- on light bouncing off of objects, or bending around objects or difffracting 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 a defining
- characteristic of our reality, but at the same time,
- when you really go down to experiment and observe with light,
- its 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 is tons of thing amazing 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,
- but light behaves as both a wave AND a particle.
- And this is, probably not that intuitive to you, because its
- not that intuitive to me!
- In my life I'm used to certain things
- behaving as a waves, like sound waves,
- or the waves of an ocean.
- And I'm used to certain things behaving like particles.
- Like... Basket balls, or... I don't know...
- Or... 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 photo-electric 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 beam
- 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 wave length [v = f * lambda]
- Now, even within, 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.
- Air particles right here
- 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 can 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 needs no medium
- Light will actually travel fastest through nothing, through a vacuum.
- And it will travel at an unimaginably fast speed: 3*10^8 m/s.
- 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 then seven times in a second.
- So unimaglably fast. And not only is this just a super fast rate,
- or 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 is a ant riding on top of that something
- moving in the same direction it would be going even faster.
- But nothing can go faster that the SPEED OF LIGHT.
- It's absolutely impossible based on our current understanding of physics.
- So its not just a fast speed it is THE FASTEST 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 8th meters per second...
- But 3 times 10 to the 8th is a pretty good approximation.
- Now within the visible light spectrum and I'll talk about
- whats beyond the visible light spectruum in a second.
- You are probablly familiar with
- colors and maybe you imagine it as a colors of rainbow,
- and rainbow 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 frequenty wavelengths,
- the violet and the blue, get refracted more, its get bent,
- it's 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 light,
- visible light, its between 400 nanometeres 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 quantuum 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 FREQUNCY 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 a part
- of a much broader phenomenom that is just a 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 electromagentic radiation.
- [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 radio to reach your car,
- the things that allow your cell phone to
- comunicate with cell towers.
- Microwaves, the thing that start vibrating water molecules
- in your food so that they heat up.
- Infrared, wich is what our body realeses and thats
- way 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 phenomenons.
- These are all the 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 frequancies of this?
- How come we only see these frequencies?"
- And I'll put these...
- 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 its illumating the earth, and if
- as a species you wanted to observe things
- based on reflected energy, a reflected electromagnetic energy,
- it is most usefull to be able to perceive the things
- where there is the most electromagnetic radiation.
- So it is possible that in other realities or other plantes
- There are species that perceive more in the ultraviolet range,
- or infrared range.
- Or 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
- of 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!
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
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This is great, I finally understand quadratic functions!
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At 2:33, Sal said "single bonds" but meant "covalent bonds."
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