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Light and the luminiferous ether | Special relativity | Physics | Khan Academy - YouTube
Channel: Khan Academy
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so let's think a little bit about waves
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and the classical notion of what a wave
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is it's a disturbance traveling through
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a medium
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disturbance
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traveling
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traveling
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through through
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a medium so what do we mean by a
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disturbance and what do we mean by a
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medium well we could come up with some
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fairly simple examples let's say that i
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have a let's say i have a little puddle
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here so that's my little puddle
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and let's say i were to drop a stone in
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that puddle so this is my stone it gets
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dropped in the puddle and we've all seen
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what happens right when it goes into the
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water that pushes that water down pushes
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the water around it out then that water
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comes back up and then you have this
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traveling disturbance
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you have this traveling disturbance
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these
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these kind of rings start to radiate
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outward from
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from where
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from where i actually
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from where i actually dropped
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dropped the pebble and you can actually
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see them you can actually see them move
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outwards you can see the crests of those
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waves move outwards so we're talking
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about a disturbance well this is that
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disturbance we the the pond was
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completely flat but then we agitated we
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pushed it down it pushed the water
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outwards and then that pushed the water
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next to it up and down up and down and
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it traveled outwards from the point of
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that initial disturbance so we have this
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disturbance
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traveling through the medium what's the
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medium here
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well the medium here is the water
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and the disturbance the initial
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disturbance was the rock disturbing that
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water but then that water disturbed the
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water around it which disturbed the
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water around it so that disturbance kept
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traveling through the medium and we can
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give other examples of this
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if i take a
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let's say if i were to take a really
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long string and hold it right there and
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that's my hand
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so when the sting the string is let's
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say it's just attached to something
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so let's say it's a really long string
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and right now there's there's some slack
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in it it's attached to a wall
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and let's say that i were to start
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moving my hand up and down so i move my
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hand up and down what's going to happen
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well if i just let's just think about if
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i just did it once if i just did it once
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what i'm going to have is this lump of
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string
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move
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from the left to the right so that's
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what it's going to look like
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at first then a few seconds later it's
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going to look like it's going to look
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like this
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it's going to look something like this
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and you're going to have this
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disturbance which is this this this wave
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this lump that i just generated it is
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going to move it is going to move
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to the right what's the medium the
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medium in this case is the string
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it is the string and what what what just
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happened there well when i jerk it when
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i jerk the string up i'm disturbing
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those string molecules right next to my
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hand and they're pulling on the string
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molecules next to them pulling on the
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string molecules next to them and so you
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then you have this traveling lump so
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once again these are both examples of
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waves disturbances traveling through
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through a medium
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now i can give other
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through a
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medium i could give other examples sound
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waves what causes sound waves well you
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have a bunch of you have a bunch of air
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particles
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all bouncing around at actually
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surprisingly fast
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surprisingly fast speeds but if you
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disturb them if you cause something to
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say compress a bunch of air particles
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right here so these air particles get
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ultra compressed some type of clap
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happens
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so these get ultra compressed well then
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they're going to bounce on the ones next
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to them and so once again you have this
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disturbance traveling through the medium
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in this case the medium is the air the
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medium is the air
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so now that we've seen some classical
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notions of ways
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let's think about something a little bit
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more mysterious and that is the notion
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of light
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we do believe light has well light
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definitely has wave-like properties
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light can interfere with each other we
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could do a whole other videos on on
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light if i were to take
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if i were to take
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some
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if i were to take a barrier like that
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with two small slits in it
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with two small slits in it and if i were
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to shine some light
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if i were to shine some light one way to
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think about is these slits are the only
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place where the disturbation the the the
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disturbance gets through and then it
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would cause the light to propagate from
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each of them like that and maybe i'm
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just drawing the crests of the waves but
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also come out from here
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and so you can see where the two crests
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where the two crests meet they're going
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to constructively
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constructively enhance each other
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they're going to constructively
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interfere and if you were to put some
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type of detector if you were to put some
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type of a detector right over here you
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would see you would see the bright
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points of the light and you would see
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the dark points of the light so light
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definitely behaves like a wave i'm just
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showing even just one example of light
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being behaving like a wave
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but if it is a wave well that means it
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needs to be or if we use our classical
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logic you would say well that means it's
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a disturbance traveling through a medium
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but what is that medium you know we have
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light coming from our sun
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we call it sunlight we have we have
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light coming i keep wanting to make it a
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actually our sun is white we tend to
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draw it as a yellowish color but that's
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just because of its what happens as it
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goes through the atmosphere so
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you have the sun
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you have the earth this is not drawn to
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scale and somehow that light is able to
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reach us over 93 million miles
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over 93 million miles what's it what's
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it going through is it a disturbance
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traveling through a medium well for a
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long time people said well it must be it
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has these wave like properties
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it's it's it seems to be traveling with
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it with a velocity the speed of light so
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there must be a medium that is traveling
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through and people theorize what this
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medium is they called it the
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luminiferous ether let me write that
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down because it's a fun word
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it's a good name for band luminiferous
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lumen
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lume
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nin i'm not spelling it luminiferous
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ether
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and it was this idea that maybe in even
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in the vacuum of space even in the
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vacuum of space
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you have this
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this substance this luminous
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luminesce ether that the light is
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traveling through that it's somehow a
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disturb it's a disturbance in
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that luminiferous
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luminiferous ether now what we're going
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to do in future videos is we're going to
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test that we're going to see if that's
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actually the case because what's
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interesting about if there is a
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luminiferous ether well we're not going
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to be stationary relative to that
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luminiferous ether in fact we're
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orbiting we're orbiting around the sun
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the sun is orbiting around the center of
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the galaxy there's no way that we're
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going to be we're going to be stationary
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relative to that luminiferous ether so
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if we're not stationary relative to that
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luminiferous ether we should be able to
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detect how light behaves if it's going
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kind of in the direction of ether or if
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it's going against the direction of the
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relative movement of the ether relative
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to us so anyway i'll leave you there
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this mystery of science you know waves
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disturbance through a medium we saw that
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with the water with the air with the
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string but what about light is there a
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luminiferous ether put a question mark
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there
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