Question: What is your favourite topic in science

Adam Washington answered on 17 Jan 2024:

My favourite topic is one I rather stumbled into in my career – Structural Colour. We’re used to thinking of colours as coming from a material. Copper is reddish, milk is white, and glass is clear. When we want to change the colour of something, we add a dye or a paint to change the material. However, was if we wanted to change something’s colour without brining in anything new.

I’m going to stop for a second here to ask you to look at your finger nails. Unless you’ve painted them, they’re likely a rather translucent whitish colour. Your finger nails are made of keratin, just like your hair and the feathers on a bird. I mention birds because of the bright white feather or a swan or the blue feathers of the Indian roller. How does keratin come in all of these different colours?

The answer is that, in the bird feathers, there are tiny little holes in the keratin. These holes are so small that they are about the same size as visible light. The light bounces off of these holes, so, instead of being mostly transparent like our fingernails, all the light bounces off of the swan’s feather and it looks a brilliant white. For the Indian roller, the holes are very specifically sized so that only the blue light is scattered (actually, they mostly reflect ultraviolet light, but we don’t see that like the birds do, so humans only notice the blueness).

Part of what makes Structural Colours so fascinating is that it can cross into so many different domains of science. There’s the physics of how the individual light particles scatter off of the holes in the material. There’s the chemistry of how these materials build the tiny holes. There’s geology and biology in learning about the various rocks and living creatures that have these colours. There’s mathematics in understanding the connections between the geometry of the material and the colour that comes out. There’s even a little bit of psychology in understanding how our minds process the colours from the lights that connect to the eye.

It even comes into medical issues. The lenses of your eyes contain collagen to provide support. If you do a quick calculation of the amount of light one collagen fibre will scatter and multiply that by the number of fibres in your eye, the math will tell you that you should be blind! The reason that you’re not is because the fibres are arranged in a very specific structure that cancels out all of the scattering and the lens becomes clear again. However, as we get older, we lose some of those fibres, creating holes in the pattern. As the number of holes increases, the structure loses its cancelling power and the lens of the eye becomes opaque. This is where cataracts come from.