Naked Engineering - Cat's Eyes and Retroreflectors

30 January 2011

Interview with

Dr Hugh Hunt, Cambridge University

Ben -   Whilst driving on motorways or busy roads at night, many of us will have noticed the small beacons of light in the middle of the road that help to position and guide us on our travels.  You probably know that these are called cat's eyes and this week, Meera and Dave have been out investigating exactly how they work...

Meera -   For this week's Naked Engineering, Dave and I are Cat's Eyejoined by Dr. Hugh Hunt from the University of Cambridge.  Now Hugh is a Fellow of Trinity College so we're here inside his office inside of the college.  Dave, why are we here?  What particular challenges of engineering are we going to be looking into?

Dave -   Well have you ever driven along the road in the dark and, especially if it's a bit misty, it's really hard to see anything driving past.  Even if someone's wearing a bright white shirt, the actual amount of light which gets reflected back to you is very, very limited.  So there's a big engineering problem of finding a way of making objects really bright when you drive past them.

Meera -   So Hugh, we have a variety of mirrors here in front of us.  What will we be doing with these to tackle the challenge of making things visible at night time?

Hugh -   Well we know for instance that the headlights from a car are very bright.  So I've got a little torch here and if I stare straight into it, it's very dazzling.  And if I have a mirror, I can reflect the light into my eyes, and it's also very dazzling.

Meera -   The light is just coming straight back at you.

Hugh -   The light is coming straight back.  If I have a piece of white paper - well, it's not at all dazzling.

Dave -   I guess a normal mirror will shine light straight back at you, but as soon as you get the angle slightly wrong, that beam is going to miss your face, and it's not going to look very bright at all.

Meera -   So the challenge there is designing something that can catch light from many angles and still make it visible back at you?

Hugh -   Exactly right, Meera.  Now one thing you might have noticed, if you've ever been into a bathroom where there are two mirrors side by side at right angles to each other, it's quite interesting to look at what happens.  So I've got a couple of mirrors here, just at right angles to each other.  Now Meera, if you look into the mirror, you will see that your reflection is at the intersection of the two mirrors.

Meera -   Yes, I can see myself at the intersection, at the joint.

Hugh -   Now what's really interesting is if you move your head from left to right, your reflection will always be at that intersection.

Meera -   That's right.  It's not moving.

Hugh -   And it's almost as if you've got a perfect plane mirror which is always pointing at you.  It compensates for the movement.  Wherever you're looking from, it always reflects directly back at you.  Now if you take a light, I've got a little torch here, and you held it by your eyes, you'll find that all rays reflects directly back at you.

Meera -   Yes and it's coming straight back at my eyes.  Actually, it's hurting a little bit!

Hugh -   This is the basic principle of what's called a retroreflector.  It reflects directly back at you and it works because, if you do the angles of reflection of the two mirrors, when you add up the two sets of reflections, the total angle of reflection is always 180 degrees.  So, whichever direction light goes into these pair of mirrors, it comes back at 180 degrees.

Dave -   So this works great if you're moving left or right, but at the moment, if you move up and down, your image moves, so it's not going to work very well like that.  How do we get around that problem?

Hugh -   The next clever trick is to have three mirrors at right angles.  So imagine you go into your bathroom and you've got one mirror on the floor and two mirrors at right angles on the walls...

Meera -   Which you've now constructed here...

Hugh - ...that I've now constructed, well where do you see a reflection now, Meera?

Meera -   It's at the intersection, but of where all the three mirrors meet.

Hugh -   Right.  Now, if you take the torch and you put that by your eyes, you should find...

Meera -   Oh, it's glaring back at me.

Hugh -   And as you move left, right, up, down, any direction...

Meera -   The light is just chasing me...

Hugh -   Always, it's quite spooky.

Meera -   So whereabouts these retroreflectors used then?

Hugh -   Well, they're used in all sorts of applications.  So perhaps some of the common ones that we're most familiar with, are reflective clothing, reflectors on the side of the road, and even reflectors on your bicycle.  They are retroreflectors and they all work, generally speaking, on the principle of this corner-cube idea, the three mirrors at right angles, and in fact, it's called a corner-cube.

Meera -   So that is quite a wide range of uses really, but Dave, another use of them that's quite commonly known is cat's eyes that we see dividing lanes on the road for us to see lane markings and essentially see when we're driving on busy roads at night.

Black and white housecat looking at the camera and exhibiting yellow tapetum lucidum eyeshine.Dave -   That's right.  In the '30s, there was a guy called Percy Shaw who was driving on the road and suddenly discovered it's really, really hard to follow the road in the mist, in the dark, and then he saw some cats, and their eyes were shining back really brightly.  If you've ever shone a torch at a cat, you see their eyes reflecting incredibly brightly.

Meera -   This is because they have reflectors essentially at the backs of their eyes.

Dave -   Yeah, their eyes acts as retroreflectors, so he attempted to copy these and then put them on the road.

Meera -   We have some of these cat's eyes here with us.  So how do these actually work, Hugh?

Hugh -   They work by taking the light coming out of our headlights and shining it directly back at us and there are some really neat things about the design of these cat's eyes.  Firstly of course, they're optically very good and they reflect the light back, but what would happen if they got covered in mud or grit and stuff from the road?  One of the really lovely things about this particular design is that every time you drive over them, there's a little rubber flap at the front, a bit like eye lashes really, and the very action of driving over these things...

Meera -   Pushes it down...

Hugh -   Pushes the cat's eyes down and clears the mud off them and it's just such a beautiful piece of clever engineering because of course, this has to be designed so that over the lifetime of these cat's eyes, you've got several million lorries passing over.  They've got to survive cold winters, snow ploughs, they've got to survive hot summers.

Meera -   So do these cat's eyes follow the main principles we've been talking about then with corner-cubes?

Hugh -   Well, there are different types of cat's eyes.  The ones that were developed in the '30s are actually more like a combination of lenses, and in fact, operate much more in the way a cat's eye actually works.

Dave -   Yeah, that's right.  If a cat's looking at something bright like a torch, the light from the torch will get focused onto its retina, so it gets an image of the torch, but the cat's eye's got a reflecting membrane at the back of the eye which reflects the light straight back the way it came, out through the lens which focuses the light, exactly straight back towards the torch which is putting the light out in the first place.

Hugh -   It's really very nice little bit of geometry and looking at how you can design a lens and a mirror, a curved mirror, a curved lens such that you get the light to come straight back the way it came in.  But it's much cheaper and much easier to make a corner cube idea.  You can just press them out of a piece of plastic, make cheap reflectors for your bikes, and those sorts of reflectors are used on the posts on the side of the road.  They're not driven over and they don't get covered in mud and grit from the surface of the road.  They're just much more like bike reflectors that are stuck under the posts.

Dave -   And you sometimes get them stuck onto the road, but they tend to be the ones which fall off within a couple of years.

Meera -   But also, along the motorways and things, you see different colours of these cat's eyes to mark out say, the hard shoulder and things like that.  How do these work?

Hugh -   It's just a coloured plastic or coloured glass.  So in the same way as if you shine a light through a piece of red plastic, the light turns red.  So if you make the plastic coloured red, it'll be red.

Meera -   So there are just such a wide range of uses for the concept of reflection.

Hugh -   Well, it's wonderful.  It's one of these things that if you actually stop and think about how they work, it's not obvious.  When you find out how they work, it's just fantastic to understand it.

Ben -   So, some basic principles of physics get engineered to help us see at night.  That was Dr. Hugh Hunt from the University of Cambridge, talking to Meera Senthilingam and to Dave Ansell.

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