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Introduction to light

Light and the electromagnetic radiation spectrum. Wave and particle-like behavior, and how to calculate the wavelength or frequency of a light wave.  Created by Sal Khan.

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  • aqualine seed style avatar for user Johnny101
    Sometimes i see gasoline on the pavement, and when i look closely i see a rainbow. How does that rainbow form?
    (514 votes)
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    • orange juice squid orange style avatar for user hi
      Gasoline (like soap bubbles) reflects multiple colors due to an effect called "thin-film interference." Basically, gasoline forms a thick layer on top of water, so light is reflected once when it passes from air to gasoline, and another time when it passes from gasoline to water. When those two waves interfere constructively, you get different wavelengths/colors, and because the gasoline has different thickness across, you get all sorts of colors.
      (78 votes)
  • blobby green style avatar for user dbuck
    What is the nature of color?
    (132 votes)
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    • blobby green style avatar for user johnnyredzin
      In a nutshell: The color we percieve an object to have, is the light that was reflected by that object. For example, a (yellow) banana is reflecting light of approximately 580 nm wavelength. It absorbs the rest (or at least the rest of the visible wavelengths).

      So the color we see is the light that is reflected. If the object doesn't reflect any of the visible light, it would appear black. A surface reflecting most of the visible light would be white.

      I hope this answers your question.
      (256 votes)
  • blobby green style avatar for user Aisha Khan
    Why is a rainbow a specific shape? Or why does it go in a certain direction?
    Since the rain droplets fill the entire space where it's raining, shouldn't that entire space be a rainbow? Sorry if this is a stupid question.
    (73 votes)
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    • leaf green style avatar for user Jabulani Mhlanga
      Light is refracted at a very specific angle. I won't go into the mechanics here, but in a raindrop, light in fact undergoes so much refraction it bounces back in the direction the original ray came from, which is why rainbows always appear on the opposite side of the sky as the sun. This means that if you stand with the sun directly behind you, the rays of light form a triangle between you, the observer, the raindrop the light is refracted from and a point on the ground directly ahead of you.

      Since raindrops will be scattered all across the sky, and you can only observe light refracted at the specific angle, the line along which the light rays travels describes an arc across the sky, and this arc is the rainbow you see.

      Incidentally, the angle of refraction is also the reason why you can never catch up with a rainbow. The angle, and hence the bow's apparent distance must remain constant with respect to your position.
      (80 votes)
  • mr pants teal style avatar for user rozina
    A few questions......

    1. how did we even get close to measuring the speed of light

    2. what are Quantum Mechanics

    (58 votes)
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    • leaf green style avatar for user Derrick Douglas
      1. The speed of light was originally estimated by astronomers hundreds of years ago through observation of astrological events, and today through instrumentation. What specific instruments i don't know. Just remember from chemistry reading about an experiment between two mountain tops in California and a laser, and I would imagine a similar arrangement in a vacuum.
      2. "Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of the dual particle-like and wave-like behavior and interaction of matter and energy." Wikipedia.org
      (35 votes)
  • male robot hal style avatar for user 19zuzaedmu
    Why is it that when I look at pictures of space, space is black? What makes it like that?
    (20 votes)
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    • piceratops ultimate style avatar for user Djordje Batic
      Earths Hubble Sphere has a verry big diameter ( 14 billion light years ) so logical thing to say is that we should see a lot of light because the sphere has a verry big volume.

      BUT, there is a thing called Doppler effect ( you can google Redshift ) which changes the frequency of wave, and makes it redder and redder until it turns infrared and we can not see it anymore.
      So with our eyes we can see only the part of space in wich the Doppler effect didn't turn the light infrared. We can "see" the infrared and cosmic radiation only by using gear, but we can't see it with naked eye.
      (10 votes)
  • leaf blue style avatar for user Xtian Torreto Factora
    4.) does light has a limit? for example a candle with light can be seen 1 kilometer...what if the sun? how far it will take to be unseen in the vastness of the universe?
    (25 votes)
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  • duskpin ultimate style avatar for user Divneet
    why do we see sky blue in color ?why not yellow or any other color?
    (14 votes)
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    • male robot hal style avatar for user Enn
      We see the sky blue in colour as shorter wavelengths of light are scattered more by the atmosphere. Blue has one of the shortest wavelengths in the visible spectrum.
      Violet and Indigo have a shorter wavelength than blue and so are scattered more but our eyes are more sensitive to Blue light as to Violet and Indigo light.
      (26 votes)
  • blobby purple style avatar for user Lauren
    Why can't anything travel faster than light? Is this just a simple fact in the world of astronomy?
    (17 votes)
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    • blobby green style avatar for user Aatman  Shah
      In a object ,the more mass ,the more energy is required by it to attain high speed.light is made up particles .the mass of photon is very very very less, so it is possible to attain such a high velocity. It is difficult to find a substance that can have mass less than photons.If we find such a substance , then it might be possible to move it with a speed greater than light
      (11 votes)
  • mr pink red style avatar for user Steve Tattersall
    It is claimed that light as a wave does not require a medium in which to propagate. How do we know that? Could it be that the propagation medium is not yet understood?
    (21 votes)
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  • hopper jumping style avatar for user Ty
    There's a thing called feedback:
    For example there is a microphone here and there is a sound system and speaker, which makes the sound louder, 6 metres away, you speak into the microphone and the sound goes through wires and out through the speakers. However, the sound from the amplifiers can go into the microphone and into a loop, getting louder and louder every time until the sound system is destroyed. Can this happen with light?
    I wanna thank Stephen Hawking for teaching That in his documentary.
    (6 votes)
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Video transcript

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.