Trapping a Beam of Light In a Loop Of Fiber Optic Cable

in my previous video I showed you the

most reflective surface on Earth that's

about

99.5% reflective but is it possible for

something to be 100% reflective well

actually it is and it's not that hard to

do this water can actually become 100%

reflective when viewed from the right

angle for example if we look at the

surface of the water from below at this

angle we can see right through it but

suddenly when we hit a certain angle

called the critical angle it becomes a

perfect mirror

and it truly isn't litting any light

through so it's really weird we can see

into it but the light can escape from it

so at this angle you can see the laser

coming up here and it's exiting the

water and shining on the background here

some of it's being reflected but not a

lot of

it so watch how bright this part of the

laser gets after I reach the critical

angle so this isn't the critical angle

but then bend it a little bit more and

now this beam here is just as bright as

this beam which means the reflection is

just as bright so it's perfect

reflection this is called total internal

reflection it happens when light's going

from a denser material with a high

refractive index to a less dense

material with a lower refractive index

but only past a certain shallow angle

will it occur in fact this is how they

make diamonds look so bright and shiny

if you angle the cuts it the right way

the light doesn't just exit the bottom

of the clear Diamond but it gets

reflected off the bottom cut Cuts as

though they were mirrors and it makes it

easy to do with diamonds because they

have such a high refractive index so the

angle doesn't have to be as shallow for

the light to reflect off it like it's a

mirror so we know that we can make total

internal reflection occur with all sorts

of different materials even glass so if

I have this long sheet of glass and look

at this laser light inside you can see

that the laser gets trapped inside of

the sheet of glass so what if we had

hundreds of feet of a small rod of glass

well then we could trap light inside the

rod of glass if it's hitting at the

right angle and that's exactly what I

have here this is a tiny Rod of glass

inside a sheath of plastic to stabilize

it the glass inside has a tiny diameter

about a tenth the size of a human hair

so in order to get my laser light into

this tube I need to focus this laser

light see how big that is way bigger

than a tenth the size of a human hair so

I need to focus that all down into a

very small diameter so I have a lens on

this one that focuses the laser light

down so it's spread out here but that's

because it's past its focal point the

focal point is right to where exits the

laser right there very small focal point

so if I plug this into my long glass

tube Watch What Happens I get a laser

light coming out the end of

it look at

that think about how many Reflections

it's going through through this entire

cable it's bouncing off that tiny little

hair of glass all the way way through

this and because it's almost perfect

reflection through this entire thing I

get about the same brightness of light

coming out of it that I'm putting into

it once it's inside the light's locked

in there it can't escape because inside

it's coming at such a shallow angle

total internal reflection occurs and

keeps the laser light locked inside of

it as if it were a perfect mirror that

it's hitting on the outside now for some

of you this might not be that surprising

to learn and you might even recognize

what I have here this is simply a fiber

optic cable these are used all over the

place in place of electric wires you can

send light through the cable instead of

electricity and then you can convert

that light signal back into electricity

to be used by computers so I'm going to

use this to convert this light signal

into an electrical signal so I turn on

my laser here you can see I have it

coming out the end and I'm just going to

plug this into

here and I'm going to measure the

voltage on here so I get 39 volt

so you can see I'm turning this light

signal into a voltage on 39 volts off

zero one of the downsides of fiber optic

cables is that since the light has to

bounce off the core at a certain angle

they can't bend too much so it can

easily curve around inside of this cable

but not if I bend it too much see the

light escapes from it now because we're

not past its critical Angle now so it

can just exit the glass there's no

longer perfect internal reflection you

can see it very clear if I just show the

center plastic part directly around the

core so now I've Stripped Away the outer

coating so now I just have the plastic

sheath with the glass core right inside

of it look how weird this looks it

suddenly just lights up when we give it

any significant Bend this is so cool so

as long as you don't bend your

fiberoptic cables too much then you can

basically have a perfect surface to

continually reflect the light through it

but then that leads me to my next

question what what if you just shine

some light in a long Loop of cable and

then removed the light and connected the

two ends together would the light just

continually go around in a circle

forever so you essentially trapped light

inside of the cable so I can connect

both ends of these so that I have a

complete Loop now so what if we were to

trap all the light inside of this Loop

okay I've got light coming out of the

end now I'm going to hurry and take it

off and plug it

in I get it

well the first problem with doing this

is then the instant that I disconnect

the light from this end of the cable to

connect the other end then the lights

already exited the other end because I

don't know if you've heard this already

but light moves really fast so there's

no way I can move fast enough to connect

the other end so if we want to trap

light in this Loop then we would need to

somehow inject the light into the cable

while it's already in a closed loop but

how could we do that so basically you

saw that when I bend the cable too much

light can escape but but if light can

escape that means the light can also

enter it as well so if I just open this

cord up so we have access to the sheath

core here then I can inject the light

into the cable so that it comes out both

ends so I'm going to put this bent piece

right in front of the bright flashlight

and I'm going to show you on my cell

phone camera here how it lights up the

center here so you can see that light is

entering from here

[Music]

you can see that I can see the white

light coming out both ends so let's try

to store some light in the loop of cable

by injecting it in the middle here now

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back to our experiment so let let's try

to inject some light into our cable here

okay we've got it connected on both ends

so it's a continuous loop now let's turn

on our 100,000 Lumin flashlight and

light up this band to send some light

into the

loop holy

cow it's really hot on my hand that is a

lot of light okay release the band so

it's straight now now that we aren't at

the critical angle anymore no more light

can escape from the cable so we've

stored it inside the cable okay we've

got the continuous loop now so let's

open it up and see if we see any light

shining one 2 3 nothing or we can also

charge it up

again then to L the light out we could

just bend

this but no light so do we actually have

light going around in loops around and

around in a circle here even the best

fiber optic cable lose about .1 DB per

kilometer of cable now that doesn't

sound like a lot but since light travels

so fast that means that in this cable

after only 1 second we only have about

10 to the -2000 as much light as we

started with so it quickly gets absorbed

into the cable and decays to nothing but

it did travel around the cable many

thousands and hundreds of thousands of

times so when we open it up we can't see

any light come out of it still but that

doesn't mean a loop of fiber optic

cables can't be use ful sometimes you

need to delay the signal in a cable for

a few Nan seconds to match up with the

RF signals in two fibers so you can put

a delay in the line that holds up the

light for a split second you can also

use Loops of optical fiber for ring

resonators for interferometers so in our

case although we did technically trap

light it quickly got absorbed this is

similar to what happens when you trap

Light Between Two mirrors it quickly

gets absorbed as well but in the case

with Optical fibers it stayed alive in

there much longer than the mirrors since

the reflection of mirrors absorbs almost

10% of light even with very good mirrors

with each hit so even our most

reflective mirror would have almost

completely absorb the light with only

about a thousand bumps which is only a

few inches If This Were a small cable so

what are your ideas to store light as

long as possible in a confined space and

thanks for watching another episode of

the action lab I hope you enjoyed it if

you did don't forget to subscribe to my

channel if you haven't yet and we'll see

you next time