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in the flat earth community her popular

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experiment is shine a laser from a low

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height across a body of water and see if

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that laser is visible on the other side

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this seems like a reasonable test if you

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put the laser and the camera both a

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three feet above the water and they are

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say six miles apart then the laser

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should be hidden behind several feet of

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the curve of the earth and yet there are

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all these videos showing people doing

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this test sometimes over much longer

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distances and sometimes with a laser and

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the camera even closer to the water and

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yet in all these videos you can see the

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laser what does this mean is the earth

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flat no of course not the laser is

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simply bending around the curve of the

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earth due to refraction we've all seen

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refraction is the bending of light

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towards more dense air generally this

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goes one of two ways there's bending

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upwards if there's some low warmer air

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like a hot road or water that's warmer

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or it will be bending downwards which is

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what happens normally as lower air is

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generally denser than higher air this

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downwards bending is amplified if the

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lower air is cooler like if the water is

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cooler than the air which is normally

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the case during the day and in the

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evening in this case with light bending

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down towards cooler denser air the

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refraction can bend the light around the

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curve and so you can see the light from

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a laser that would normally be hidden

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but if we got these two cases like

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bending up and light bending down then

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why do the tests always show the laser

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visible well they actually don't if you

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follow these tests you'll see people are

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constantly unable to spot the laser

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which they quite correctly blame on the

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weather for example there's a recent

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test in Brighton where the light was

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bending upwards they could not see the

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laser now we've got another test asking

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can the intense source of a laser one

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meter above the water be recorded 9.5

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kilometres away by a camera one meter

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above the water level and the answer is

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yes

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of course it can why are you even doing

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this experiment don't you watch YouTube

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we've seen time again that lasers are

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often visible over water when

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geometrically they should be hidden

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biting the curve so there should be no

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surprise what's actually going on when

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you see a laser when you think you

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shouldn't well there's two things to

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consider a refraction and beam

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divergence no I wrote a refraction

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simulator and the refraction simulator

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basically does what it says it simulates

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refraction we have a picture and we have

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a temperature profile and and we've seen

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relate where all the beams of light go

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and this shows us the resultant image

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and you can move the temperature profile

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around and you can make it colder or

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warmer and you can see what effect that

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has and I have a setup

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for the Bedford level which is the site

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of the experiment I've referenced and I

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can load that here Bedford level boat

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this is a bridge which is six miles away

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that's a gain about is about four miles

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away I'm actually going to move

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I'm going to select him here man in boat

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I'm going to move him out so he's also

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at six miles so it's just about where

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the bridges okay Cece is being squished

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there because I'll see him

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now the Bueller here is only four feet

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above the water so really you shouldn't

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be able to see when in fact if we turn

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off refraction you'll see that he is in

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fact down below the horizon we can move

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them around move them back here and you

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can see he's actually there moving back

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out to six feet so I'll just put that in

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exactly six miles away and we can raise

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the viewer height so you can see what's

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going on we can go higher so we can

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actually see the guy down by by the

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bridge over there but let's move him

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back down so one meter that's about

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three feet so let's go down to about

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three feet and recently I added lasers

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to the mix to any refraction back on

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here and like I switch is on down here

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by default it will set a laser etched

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the camera so here's where the camera is

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here's where the bridge is the laser is

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a four feet here their cameras are three

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feet we can move the laser hide other

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places at five feet we can move the

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laser down we can move it up and this is

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kind of boring because we're just

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shining towards the guy in the boat

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let's flip the laser direction which

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will put the laser we'll put the laser

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at the the bridge show the guy at the

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boats here has to miss it at the

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furthest object I'm going to reduce the

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power of this laser is a bit too bright

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and you can see here's the laser in the

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boat it's at one point seven feet and

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I'll make that three feet so it's same

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as in the experiment so we've got a

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laser six miles away we have the viewer

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the camera a three feet we have the

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laser at three feet and the quite

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visible so why is this this this this

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thick green line here is the line of the

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laser which you know you see you're

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looking at it here it looks like it's

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actually going too high you wouldn't get

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to see it and what's actually happening

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though is the laser spreads out it

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spreads out by one milli radiant which

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is a thousandth of a radiant which is

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it's just a small fraction of a

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a degree which isn't really very very

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much it kind of looks like a lot here in

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this side view but that's because this

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side view is very very very very

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compressed horizontally or stretched

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vertically depending on which way you

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think of it and if I turn on the the

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zoom and if I set the zoom factor to one

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which means this is the actual actual

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site you'll see things look very very

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very flat the the earth of course you

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know it doesn't cook very much because

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it's so big so over six miles is hardly

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any curve it looks pretty much flat we

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can zoom in to reveal the curve and we

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can see this is the spread of the laser

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beam if I change the angle of the laser

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you'll see the green lines in the middle

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on those two green lines one on either

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side this is showing the divergence of a

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laser as I say it's been magnified

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because we were magnifying the curve so

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we can see what's going on so if I go

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back down to one it would actually look

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like that which you know it looks like a

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straight line but it's actually

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diverging very very slightly anyway so

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this laser beam starts over here and it

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goes in this direction and the bottom of

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it spreads to here and the top spreads

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to here so if it's angled down a little

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bit so it's down like it was before the

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top of the laser beam could be going way

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over the camera but the bottom of the

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laser beam is just touching the can we

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just managed to make it around the curve

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of the earth and you can see down here

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you can just see that beam start to come

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in so the divergence of the laser beam

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actually has quite a quite a significant

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effect and that it makes it very very

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easy to see the beam that kind of no

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matter what direction you're pointing it

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in like up or down because there's

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always going to be some light path which

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goes over just over the curve of the

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earth and we just get rid of this real

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quick just move it over here and I'll

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move this into a so you can see it a

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little bit better and I'm going to zoom

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in a bit more

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so here's the camera and here's where

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the laser is both there were three feet

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above the ground and there's even though

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the laser beam is shining up like this

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there's still a light path which goes

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from the laser to the camera and you can

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actually see that by moving this around

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here until we get to the laser so this

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yellow line here is a path that the

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laser takes now if we zoom down a little

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bit more here what the zoom taper tilts

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down a little bit more you'll see the

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laser will actually get brighter as the

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center of the beam goes closer and

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closer now why is it refracting around

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which because the the water is cooler

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and I've got this little graph here

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which shows this very very slight cool

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water then slightly warmer air and then

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just moving this is hardly anything this

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is about a tenth of a degree it really

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really does not take very much at all to

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get the actual results that we're

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looking for here now of course if the

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the air was you know even warmer then

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you get even more refraction or it

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becomes more visible but if the air is

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just a little bit cooler than the water

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the water is warmer then we don't see

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the laser so very very small variations

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can have quite a big effect we can see

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the laser here move this over here and

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you can't see it yeah you can see it you

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can't see it and this I'm just moving

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basically about a tenth of a degree

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either way so and it's only in the

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bottom ten feet or so and it could be

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even less I can focus down on the bottom

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like one or two feet and just very very

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small very very small adjustments in the

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temperature so because you don't

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actually know the temperature all the

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way along this path all the way along

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this six mile path it's actually quite

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difficult to figure out what's actually

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going on and actually what you see in

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terms of what the refraction is a better

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indication of what the temperature is

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along that path than the actual

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measurements you would make say let's

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start on the end of that path now you

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see here I put the laser in the boat and

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the reason I've got this boat here is

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the original rope

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I'm experiment back in the 1800's didn't

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have lasers back then of course you know

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which they just had guys in boats so

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what they did is they got a guy in a

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he started out here and he just rode his

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boat all the way over to the bridge and

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they looked at him from a telescope set

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3 feet away it might be 2 feet above the

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water line and they found that

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actually see on the hallway and this was

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because of course of refraction it's

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exactly the same thing here very slight

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difference in his slightly cooler water

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or much cooler water in the some cases

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let's you see around the curve of the

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earth because it's bending the light

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down so that wasn't a very good

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experiment back in the 1800s and the

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laser experiment right next to the water

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is not a very good experiment either so

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what should you do instead well you

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should use bigger things and you should

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use higher viewpoints get away from the

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random deviations of the laser being a

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few feet either way above above the

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surface of the water just grazing the

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horizon and try to look at the bigger

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picture use mountains and use tall

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buildings for example can you see all of

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Catalina Island from Santa Monica no you

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can't can you see all of San yaqeen no

271
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Jackie no mountain from Malibu no you

272
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can't can you see all of Toronto from

273
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across the lake no you can't can you see

274
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the curve of the earth from a plane from

275
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35,000 or 40,000 feet yes you can so

276
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these laser tests are fun and

277
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interesting demonstrations of refraction

278
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but the very misleading if you pretend

279
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refraction isn't happening to get the