If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



Sal Khan 创建




let's say I'm over here I'm gonna do two scenarios so I'm an observer over here this is me this is me and then maybe even better I should just draw my eyeball because we're going to be observing light so I'm just going to draw my eyeball so this is me in the first scenario and then this is or this is one of my eyeballs and then this is one of my eyeballs in the second scenario now in the first scenario so let me draw it so in both scenarios we're gonna have an object we're going to have some type of source of light but in the first scenario I am relative to me the source of light will not be moving while in the second scenario the source of light the source of light just for the sake of discussion just for fun we'll be moving at half the speed of light unimaginably fast speed but let's just assuming it is so it's moving at it has a velocity of one half the speed of light one half light speed light speed away from me light speed away away from me who is the observer now let's just imagine what would happen they're both emitting light so and they're both gonna start emitting light at the exact same time and what and when they start emitting light they're both at the exact same distance from my eye the only difference is is that this is stationary relative to me while this is moving away from me at half the speed of light so let's say that after some period of time that the light wave from this this source reaches my eye so and then it looks something like this I'll try my best I'll try my best to draw it so let's say I have I want to draw a couple of wavelengths here so let's say that's half a wavelength that's a full wavelength that's another half a full wavelength another half full wavelength and then a half and then a full wavelength so let me see if I can draw that so it would look it would look like full wavelength full wavelength full wavelength this is not easy to do and then you get another full wavelength so it would look something like that with the actual waveform and so that this is just the front of the way for is just getting to my eye and then my eye and then as the wave forms keep going past my eye it'll pert my eye will perceive some type of a wavelength or frequency and perceive it to be some type of color assuming that we're in the visible part of the electromagnetic spectrum now let's think about what's going to happen with this source so the first thing is is that the front of the waveform is going to reach me at the exact same time one of those neat and amazing things about light traveling in general or especially in a vacuum it doesn't matter that this is moving moving away from me at half the speed of light the light will still move towards me at the speed of light it's absolute doesn't matter if this is going away at point nine the speed of light the light will still will still travel to me at the speed of light and it's very unintuitive because in our everyday sense if I'm if I'm moving away from you at half the speed of a bullet and I shoot a bullet the bullet will only move towards you at it'll kind of that that half of its velocity will be subtracted and it will only move towards me at half of its normal velocity relative to whether it was stationary but not the case with the light so with that out of the way let's think about what the waveform would look like so by the time by the time the light reached here we need to think that me actually redraw this over here let me redraw this eyeball let me redraw this eyeball right over here so this is me again so by the time the light reaches my eye so they both started emitting the light at the exact same time this guy has traveled half this distance right if it took light if it took light a certain amount of time to get this far this guy will get half as far in that same amount of time so by the time the light reaches my eye this guy will have travelled about half that distance so he would have traveled about that far but just so they started emitting the light at the same time so that very first that very first photon if you view light as a particle will reach my eye at the very same time as the very first photon from this guy so the waveform is going to essentially be stretched so instead of having so we're still going to have we're still going to have see we're going to have one two three four full wavelengths but they'll now be stretched let me see if I can draw four full wavelengths so then let me cut this in half over here let me cut each of those in half so each of these are going to be a full wavelength and then they're gonna have a half wavelength half wavelength in between and so the waveform is going to look like this is going to look like this try my best to draw it this is the hardest part drawing this stretched out waveform and there you go it's going to look like this and so when it gets to my eye my eye is going to perceive it as having a longer wavelength even though from the perspective of each of these objects if you're traveling with each of them the the frequency and the wavelength of the light emitted is the same the only difference is this guy's moving away from me or I'm moving away from it depending on how you want to view it while I am stationary or it is state while in this first case the observer and the source are both stationary now in this situation what's my I going to say well my eye will will get each of these successive pulses or each of these successive wave trains and it's going to say hey there's a longer wavelength here there is a longer wavelength longer wavelength a perceived longer wavelength let me write that perceived a perceived longer wavelength here and also a perceived a perceived lower frequency a perceived lower frequency so what would that do to the perception of the light let's say that this is let's say that this is green light so if we were a stationary observer it would be green light so let's look at the electromagnetic spectrum I got this off of Wikipedia so if I were stationary towards with the observer we'd be in the green light part of the spectrum so at five a 500 nanometer wavelength but if all of a sudden because the object is moving away from me at this huge velocity it's the perceived wavelength becomes wider so from my perception it's going to have a wider wavelength and you can see what's happening it will look redder it will move towards the red part of the spectrum and this phenomenon is called redshift this is called redshift this is red red shift and I've done a bunch of videos in the physics playlist on the Doppler effect and over there I talked about sound waves and the perceived frequency of sound is something travels towards you versus away from you that's the exact same idea this is the Doppler effect applied to light and the reason why that the Doppler effect works for light traveling through space and for sound traveling through air is because the sound wave in air regardless of whether it's moving away whether the source is moving away or towards you the sound wave is going to move at the speed of sound in air at a certain pressure and all of that and light is the same thing but in a vacuum it will always it will always regardless of the source regardless of whether what the source is doing the actual light wave itself will always travel at the same velocity the only difference is is that it's perceived frequency and wavelength will change and now the whole reason why I'm talking about this is you can use this property of light that it gets redshift to see whether things are traveling away or towards you and you know people talk about redshift because frankly most things are traveling away from us and that's one of our the reasons why we why we tend to believe in the Big Bang the opposite if something is traveling towards me and relative at super high velocities then we would have something called and you don't hear the word it would be violet shift the frequency would increase the frequency would increase so it would look bluer or more purple now the other thing I want to I want to highlight is this redshift phenomenon this this idea it doesn't apply only to visible light so could even apply to things that we can't even see so it would only it would become redder but it's not like you can even see it could even be apply to things that are even more red than red so maybe it's a microwave that is being emitted but because the source is moving away from us so fast it could be perceived as an actual as an actual radio wave and actually I should have talked about this when the video on the microwave background radiation is that we're perceiving it as microwaves but the sources were moving away from us they were being read shipped so they were not actually emitting microwave radiation just what we would we observe and this was is actually what would be predicted based on the Big Bang is the act is actually microwave radiation so anyway hopefully that gives you a sense of what redshift is and now we can use this tool to to explain why we think many many things are moving away from us and now let me just actually make sure you get that idea if I have two objects let's say that these are Suns let's say that these these are both Suns or both galaxies either way and because of other properties and I won't talk about them right now we know that they are probably they are probably emitting light of the same color they're probably emitting light of the same color because we know other properties of that of that star or of that galaxy now if what we actually perceive is that this one looks redder to us than this the one then we know that that it is we know it is traveling away from us and the redder it looks the more its wavelength to spread out relative to this other star the faster we know that this is moving away from us