How does a white surface reflect light?

13 April 2008

Question

My daughter wants to know why it is that white surfaces reflect light and black surfaces absorb light?

Answer

Chris - Well, if you think about what light is, light's a wave. It's an electromagnetic wave which wiggles its way through the atmosphere, through space. When it hits something transparent the wave hits that substance and it goes through it. As it goes through it the wiggling of the wave makes the particles in that substance wiggle as well. That includes the electrons. If the substance is transparent the wiggling of the electrons regenerates the wave of light goes through the substance, albeit with a time delay, which is why the light slows down a bit on its way through. When it comes out the other side it's recreated again with no loss of energy. What about if the substance isn't transparent, in other words, if it's opaque? Well, of course it reflects light and that's gives it its colour. If it reflects no light, it's black.

We recently featured on this show the darkest substance in the Guinness Book of Records ever. Pulickel Ajayan produced this substance. He's at Rensselaer Polytechnic. The way he did that was by producing these nano-tubes: a forest of bamboo-shaped nano-tubes. These are tiny skeleton tubes like straws of carbon. They're literally thousands of times thinner than a human hair. By making a sea of these things, when light goes down into this, ricochets off a nano-tube and bounces into another one it just gets lost. It gets trapped inside so nothing gets reflected. That makes the substance very, very dark. When light gets soaked up by a substance, basically what it's doing is all the energy (the vibration of the light) is making the atoms in that substance vibrate. It's making heat. That's why solar cells that are made of black stuff get warm, because they're soaking up the light energy and radiating very little out. When a substance is radiating other colours what's happening is that some wavelengths of light, some wiggles are absorbed and they turn into heat in that substance whilst others are reflected. When they're reflected, basically the wiggle the wavelength imparts to the material creates another light wave of the wavelength of the colour that you see. That's why it reflects light of that particular colour. With white the substance is very good at reflecting all wavelengths of light which is why if you add all the wavelengths of light together you see white. That's why it looks white. That's why water is clear when you shine light into it but snow crystals are white because with water light can pass straight through. With snow crystals light bounces around all over the place so all the light gets returned to you so it looks white. Helen - So it's all about what different substances are made of and how they vibrate?Chris - Yes, the different substances will soak up different light of different wavelengths, different frequencies but not others. Different colour lights have different frequencies to each other and some will be soaked up by the surface and others won't.

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