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Current time:0:00Total duration:9:37

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 it is 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 being sensed by our eyes and then sending signals into our 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 or and well actually there's 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 but 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 am used to certain things behaving like particles like like basketballs or 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 in how you observe the light so when you observe it as a particle and this comes out of 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 of a train of particles moving at the speed of light which I'll talk about in a second and we talk we call these particles we call these particles photons if you view light in other ways and you see it even when you when you see light being refracted by a prism here it looks like it is a wave it looks like it is a wave and it has the properties of a wave it has a frequency 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 what's even within even if you ignore this this particle aspect of light if you just look at the wave aspect of the light it's still fascinating because most ways 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 writing out our 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 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 these part this this energy traveling through a medium and I'm I'm used to visualizing waves in that way but light needs no medium light needs no medium it note lead 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 8th meters 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 or it would take light less than a seventh of a second to travel around the earth or would travel around the earth more than seven times in one second so unimaginably fast and not only is this just a super fast rate and this is one 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 bead not just known to physics but possible in physics so once again something very unintuitive to to us in our everyday realm we always that okay if something is going at some speed maybe if there was an ant riding on top of that something 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 it's absolutely impossible based on our current understanding of physics so it's not just a fast speed it is the fastest speed it is the fastest speed speed possible fastest speed possible and this right here is an approximation it's actually two hundred ninety was two point nine nine something something times ten to the eighth meters per second but three times ten 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 you maybe you imagine them as the colors of the rainbow and rainbows really happened because the light from the Sun the white light is being is being refracted by these little little water particles and you can see that and maybe 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 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 gets refracted more it gets bent its Direction gets bent more than the low frequency wavelengths then the Reds and the oranges then the Reds and the oranges right over here and if you want to look at the wavelength of light they're a 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 higher frequency means higher energy 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 then it's just the part that we happen to observe and if we want to broaden the discussion a little bit light is just or I should say visible light is just really part of the electromagnetic spectrum so light is really just electromagnetic radiation electromagnetic magnetic 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 allow your your cellphone to communicate with cell towers microwaves the things that that 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 will give you sunburn x-rays the 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 can we only perceive certain frequencies of this how can we only how can 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 it's inundated the earth and if as a species you wanted to observe things based on reflected energy a 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 on 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 in the part of the spectrum where the Sun just depends 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 the thing is any of the 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 you're already at the leading edge of physics thinking