- This video is sponsored by Nord VPN.
If you look up close at a bicycle reflector
you'll see this sort of hexagonal pattern,
but it warps and changes into other things as you tilt it.
And there's a really clever reason
for the geometry that you see here.
The purpose of a bicycle reflector
is to make you more visible to car drivers.
And one way to do that passively
is to reflect more of the light that falls upon you.
And if that's what you're trying to do
then maybe just cover yourself in lots of tiny mirrors
like a human mirror ball,
because mirrors are really reflective.
The problem is, mirrors work in this really predictable way.
The angle of incidence
always equals the angle of reflection.
In other words, you don't have much control
over where the reflected light goes.
What you really want
is for most of the light being reflected off you
to end up in the eyes of the car driver.
So how do you achieve that?
Well, if you take two hinged mirrors
and set them at 90 degrees to each other like this
or a quarter tau radians,
then when I shine a laser into these two mirrors,
the returning beam is always at exactly the same angle
as the incoming beam.
It doesn't matter how I change the angle
of the incoming laser beam,
the outgoing beam will always match that angle.
I could put together a geometric proof
to show that it's always true for any angle
or for the purposes of this video,
I could just do what I like to call a proof by,
"Look there you go, doing it."
You may have spotted that
this only really works in two dimensions
because look, if I tilt the laser pointer in this direction,
actually it doesn't work anymore.
The returning laser beam is not at the same angle
as the incoming laser beam.
To fix it for this extra dimension,
I need a third mirror that is perpendicular
to the first two, like this.
And see, now it doesn't matter
what angle I point the laser at these three mirrors,
it will always come out at the same angle that it went in.
This geometry that you end up with
is the corner of a cube, see?
So this is called a corner cube reflector.
And it's an example of a retro reflector
which is just any reflector that sends light
back in the same direction that it came from.
And you can achieve this with a prism as well, like this.
So light goes in through that front surface
and the rear surface has these three perpendicular faces.
It's the corner of a cube.
So the light that comes in
bounces around on those three surfaces
comes back out in the same direction that it went in.
On this prism, those three back surfaces
have been coated with copper to make it more reflective
but it would mostly work just by total internal reflection.
And that's the working principle behind bicycle reflectors.
I've taken the back off this one
so you can see what's going on on the underside.
Actually, it's still not that easy to understand
through the camera, but look, if I spray paint it
hopefully that makes it a bit clearer.
Can you see all those spikes
in the shape of the corner of a cube?
So it's just like the backside of this thing
except it's tessellated to fill up the space.
We've actually put reflectors like this
on the surface of the moon.
And we use the travel time of laser light
to figure out how far away it is.
Like, is it slowly creeping up on us in the daytime?
It's behind me, isn't it.
So think about the light coming from the headlamps of a car,
it shines onto the bicycle reflector,
it bounces around inside all those tiny little corner cubes
and comes back out again
straight towards the lamps of the car,
not towards the driver's eyes.
But it turns out that actually
the geometry inside here isn't perfect,
so you get a little bit of a spread
of that returning light and it's enough to reach up
to the level of the driver's eyes.
To illustrate the point, I've put a torch
just below the lens of my camera,
in the same way that the headlamps of a car
are just below the eye line of the driver.
And look at the effect it has on the bicycle reflector.
It makes it incredibly bright in the lens.
And you know, I don't have to get the angle just right.
It'll return that light really brightly regardless.
And that's because the returning light
is spreading out slightly
because the geometry isn't perfect.
And a lot of it ends up in the lens of the camera.
I'll keep that torch on
while I show you this reflective fabric.
This works in a slightly different way.
So it's not covered in the corners of cubes,
instead, it's covered in tiny beads.
You can see that under the microscope here
and they work the same way as cat's eye reflectors
that you find embedded in roads.
So this is what Hi Vis clothing is made of,
it's also what anti-paparazzi clothing is made of
because when the flash of a camera goes off
the camera then tries to adjust for exposure like this.
So look, to get the fabric looking normal,
my face has to be really really dark
and the camera does that automatically
and it ruins the picture.
If you've ever wondered why some street signs
seem inordinately bright at night, in most cases
it's because they're covered in these teeny tiny beads
and they're reflecting the light back to your car.
But anyway, I digress,
this video is about corner cube reflectors,
and it's important that each individual corner cube
isn't too small.
It needs to be substantially bigger
than the wavelength of the light
that you're wanting to reflect.
Otherwise, it will just scatter that wavelength.
Which is why radar reflectors look like this
because radar might be a few centimeters in wavelength.
I found this one attached to Waterloo Bridge
and actually this one is reflecting radio waves
back to a satellite.
You also often find them on rescue boats
where you want them to be really visible to radar.
Corner effect is gonna even be created by accident
by an overhanging balcony
or two walls meeting at right angles like this.
It creates a corner reflector for sound
and anywhere I stand in this area,
I can hear my voice coming back to me.
And it's really weird.
You might now have worked out
the answer to the question posed in the title of this video.
The reason you won't find a right angle
on a staff airplane
is because they want to avoid
accidentally creating corner reflectors.
You don't want to send radar waves
back to the detector.
Brilliantly, you can use this principle
to build smaller decoys.
This is an AMD-20 Quail and it's about four meters long.
It's designed to be a decoy for a B-52 bomber,
which is about 50 meters long.
But if you look at B-52 bomber
it doesn't have much in the way of right angles.
It's not that reflective to radar, but the decoy does have
some internal reflecting right angles.
So both of them appear to be just as bright to a radar
even though one is much smaller than the other.
My favorite example of corner cube reflectors
is actually this one that you've seen already
because you see these all over the place
on the London underground, especially at the moment
at Paddington Station and Bank Station.
But why are they there?
I'm no expert at transport infrastructure,
but I know someone who is.
So if you want to hear the rest of this story,
head over to Geoff Marshall's channel,
where we work it out together in his latest video.
I'll put a link to that video in the card there,
it'll also be in the description
and I'll put it on the end screen as well.
Geoff Marshall's channel is really good,
so you should subscribe there as well.
Thanks to Joey Cook, who gave me the idea for this video
in a comment to a previous video.
I hope you like it, Joey.
So if you're out on the roads, it's important to be visible
but if you're out on the internet
you might want to think about being less visible with a VPN.
Yes, that's how I'm doing the segue.
Look, I'm really grateful, again, to Nord VPN
for sponsoring this video.
Let's get technical this time.
When you have a VPN installed on your computer,
like Nord VPN, it will encrypt all the traffic
that's leaving your device.
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peer to peer services even.
And what that means is
those services don't know who or where you are,
which means that if there's any
geo-restricted content that you wanna see,
like maybe, you know,
this video isn't available in your country,
you can set your VPN
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The other side of it is, your internet service provider
or like the coffee shop whose WiFi you're connecting to
can't see who or what you're connecting to.
So it just gives you this extra layer
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If you go to nordvpn.com/steve
you'll get 75% off, which is just $2.99 a month.
And there's a 30-day money back guarantee.
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Why not have a look at it today?
The link's also in the description.
I hope you enjoyed this video.
If you did, don't forget to hit subscribe
and I'll see you next time.
(upbeat music)