Bernie's Basics

Darker when wet

We've all been embarrassed by a wet patch. And that tell-tale patch is dark for the same reason things look darker in low light - less light is travelling from them to our eyeballs.

We see things because light coming from them reaches our eyes. Most things don't make their own light, so what we're seeing is light that's hit things and either bounced off their surface, or bounced around inside them a bit before heading towards our eyes.

The more light that makes it to our eyes, the brighter the thing we're looking at appears. If you dim the lights, things look darker (and ten years' younger) because less light is hitting them, so there's less light to bounce back into our eyes. But spill a glass of water on your shirt and the wet spot looks darker. The same amount of light hits all the fabric, but the water makes it harder for the light to bounce its way back out to us.

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The speeds of light

We think of light as waves that travel in straight lines at a constant speed — 300,000 kilometres per second. But that's only true for light travelling in a vacuum, like empty space. Whenever light has to travel through anything with actual molecules floating around in it — like air, water or fabric — it slows down.

The slowing down happens because whenever light bumps into an atom or molecule, it gets absorbed and spat out on the other side. And all that absorbing and spitting takes a tiny fraction of time, so the light passes through any material more slowly than it would through empty space.

Air doesn't have too many molecules to bump into, so it only slows light down a tiny bit (0.03 per cent). But water has way more molecules than the same volume of air, so light really has to work the room when it goes through the wet stuff — it's 25 per cent slower in water than it is in space.

And fabric is even more dense than water, so light slows down by a massive 33 per cent travelling through your t-shirt. The speed of light through a material compared to its speed in a vacuum is called its refractive index. The refractive index of empty space is 1, and air is 1.0003. Water has a refractive index of 1.33, and fabrics are all about 1.53.

But it's not the speed that light's travelling through water, fabric or air that makes wet things look darker — it's what happens when light changes speed that does the trick. And light changes speed whenever it moves from one refractive index to another. If a beam of light moves from air into fabric, it has to slow down from 300,000 kilometres per hour to 225,000 kilometres per hour instantly. And putting on the brakes like that makes light do the equivalent of an electromagnetic skid. It bends.

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Light gets the bends

The bigger the change in refractive index when light moves from one medium to another, the bigger the change in speed, and the bigger the angle that light bends at. So when light passes from air into fabric it bends at a much bigger angle than when it goes from water into fabric.

And light doesn't just bend once when it goes from, say, air into fabric. Clothes are made up of fibres and air. Light passing through a t-shirt is going to constantly move in and out of the fibres and the air, and it bends every time it does. Because the refractive indexes of air and cotton are so different, the angle of the bend is pretty big. All those big angles mean that a lot of light ends up bouncing back out of the fabric — and some of it will head straight for your eyes.

When your clothes are wet, all those air gaps get filled with water. So when light hits a wet patch it's moving in and out of water and fibres, not air and fibres. The refractive index of fabric is a lot closer to that of water than it is to air, so the light doesn't change speed quite as dramatically going between water and fibres. And that means it bends at a smaller angle when it goes from one to the other.

It's those small angles that are behind the darkness of the wet spot. With small angles, it takes a lot more bends for light to turn right around and head back out to our eyes, so more of the light ends up travelling forward into the fabric. And that patch looks darker.

Spend a few minutes under a hand dryer or sunshine, and you'll see the t-shirt lighten up again. As the water evaporates, air comes back into the water/fibre mix, and brings some bigger angles to the light, so more of it makes it back to our eyes. And we can face the world with crisp dry confidence again …

Thanks to Prof David Jamieson from the School of Physics at The University of Melbourne.

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