HELEN CZERSKI
Why black cabs make pretty good mirrors
HO WOULD HAVE thought that staring at a taxi could be interesting? I live in London and cycle to work every day, so I’m very used to waiting behind black cabs at traffic lights. Last week, I was idly watching the distorted reflection of my bike in the glossy black paint of the cab sitting in front of me, and it suddenly occurred to me that this is a very odd thing to do. We know that black surfaces are black because they absorb all colours. But I could see the colour of my top and the handlebar tape. This shiny black surface was a pretty good mirror. At the next set of traffic lights, there was a white car in front of me, and my reflection wasn’t nearly as clear. But white surfaces reflect all colours equally – surely necessary for a mirror. What’s going on?
Glossy surfaces are glossy because they reflect light in a very organised way. Light that arrives at the surface from a particular angle is reflected away at exactly that angle on the other side. It’s like bouncing a ball off the ground – bounce it straight down, and it comes straight back up. Bounce it at an angle, and it bounces off at that same angle. So glossy paint is reflective because it’s got a very smooth top surface that light bounces off, and it reflects all colours equally. But if it reflects all colours, how is it also black? The trick here is that the surface splits the incoming light. Some light is reflected perfectly, barely touching the surface. But some travels down into the paint itself. Down there, all colours are absorbed. So I could see the black taxi because it was perfectly reflecting some of the light around it, and perfectly absorbing the rest.
As I pedalled further along the road, I thought about shiny white objects such as plastic, marble and white glass. No amount of polishing will turn those into mirrors. I watched my reflection in each white car that went by. Light coming from my bright red cycling jacket was hitting the surface and being split, so that some light waves got to explore the inside of the paint layer. Inside the paint is a mini house of mirrors – lots of particles that bounce light around like pinballs. Eventually the light bounces out of the paint, but it leaves in a random direction. And this is why I found it harder to see my reflection in the white cars. I could see the perfect reflections, but they were mixed in with all the random light rays from all sorts of other directions.
A red car went past. This time, not all light that got down into the paint was getting out. Pigments in the house of mirrors were absorbing all the colours that weren’t red, and only red light got to escape.
What the taxi made me think about was that the light from a painted surface comes from two entirely separate places. To get at least some perfect ‘mirror’ reflection, all you need is a smooth surface. What’s underneath is irrelevant. No perfect mirror can have a colour, because no light ever gets beneath its surface. Therefore, there are no clues to what’s inside. But colour is all about what’s inside the paint, and some light will always sneak below the paint surface. It’s only this light that bounces around and is filtered before it comes out again, giving the object its colour. Our brains are so adept at interpreting shiny, colourful objects that we almost never notice that they’re doing it.
I love the idea that when you’re seeing a coloured object, you’re seeing inside it, even if it’s only a little way. And now I’ve got something to keep me entertained in traffic jams!
“Our brains are so adept at interpreting shiny, colourful objects that we almost never notice that they’re doing it”