Is black a colour?

We put Martin's question to Caroline Steel...

Caroline - Black is just the absence of light really. It sort of seems to be a colour to us because we can differentiate it from other colours, but a black object is actually an object that absorbs all colours of the visible spectrum, that's all frequencies of light. So say this black microphone absorbs red, blue green, all of the colors, so to us it just looks like the absence of colour, which is black. So when we shut our eyes we see black as no light's getting in.

The opposite of this would be white. A white object reflects all light and when we can see all frequencies of visible light, they combine to make sort of a white colour, although colour would be a slightly misleading word.

Chris - We had Pulickel Ajayan on the programme about 2008, I think it was. And he published this paper that said the new black, and said that he'd made the blackest substance ever made. And he sent me a picture of it and I actually had this disc of this stuff and the industry standard of what we call black. And his stuff, he says, is thirty times blacker than black. It seems paradoxical that something could be blacker than black, but it really was dramatically darker.

And their technique was exactly as you say Caroline. The reason something has a colour is because it's reflecting light off it's surface. His approach was well if we want to stop light being reflected off a surface - how do we do it. They made a layer of carbon and had this bamboo like forest of carbon nanotubes sticking up like bristles off of the surface. And their rationale was any photons of light that go in there will ricochet around and just get lost in these carbon nanotubes and never come out. It just basically turns the light into heat in the surface, and that's why it's so black. But no, an amazing discovery.