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