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Why do things look black and white in moonlight?

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Most of what we know about the world around us comes from seeing. Vision is such a useful sense because photons emitted by the sun stream down to earth all day long and reflect off objects or are absorbed by them. Materials that reflect more photons appear lighter to our eyes, while materials that absorb more photons appear darker. Because photons travel at the speed of light, the stream of photons reflecting off an object gives us practically instantaneous information about it, even if it is far away from us. The only other sense that gives us information about far distant objects is hearing.

opsin protein

The structure of an opsin protein. The corkscrew shapes represent alpha helix domains in the protein structure. The bound retinal molecule is just visible in front of the orange alpha helix and behind the two green ones.

We can see because the retinas in the back of our eyes contain cells called photoreceptors, which can detect the presence of photons. Photoreceptor cells can sense photons because they contain molecules of retinal that change shape when they absorb a photon. The retinal molecules are bound to proteins called opsins, which change shape when the retinal changes shape. This triggers a cascade of molecular events in the photoreceptor cell that alters the release of neurotransmitter molecules by the photoreceptor, thus sending a neural signal to other cells in the retina.

Our opsins have evolved so photoreceptor cells are most sensitive to photons in the range of wavelengths emitted by the sun. What we call visible light has wavelengths from 400 to 700 nanometers (a nanometer is one billionth of a meter), because when retinal is bound to opsins, it doesn’t readily absorb photons with wavelengths below 400 nanometers or above 700 nanometers. Photons with wavelengths above 700 nanometers are  in the infrared range, and they’re bouncing off objects all around us, but we can’t see them because they have no effect on the retinal molecules in our photoreceptors.

The different colors that we see are simply photons of different wavelengths within the 400-700 nanometer range of visible light. The colors in a rainbow from blue to green to yellow to orange to red correspond to photons having a range of wavelengths, from shorter to longer.

We can distinguish these different colors because the photoreceptors called cones come in three types containing three different opsin proteins. Those are called L, M, and S opsins because they interact with retinal so it preferentially absorbs photons having long wavelengths, medium wavelengths, and short wavelengths. Red light is absorbed best by cones containing L opsins, while green light is absorbed best by cones containing M opsins and blue light by cones containing S opsins. The visual circuits in our brains compare the neural activity triggered by these three different kinds of cones to distinguish between slightly different colors, like tangerine vs. pumpkin.

The blue, green. and red lines show the range of wavelengths of light that are absorbed by S, M, and L opsins. The dashed line shows the range of wavelengths absorbed by the rhodopsin proteins in our rods.

So why do we see these colors only during the day? Because cone photoreceptors aren’t sensitive to very dim light. The density of photons at night is so low that it has virtually no effect on any of the three different types of cones. Another type of photoreceptors called “rods” are the only ones responsive to dim light, and there’s only one type of opsin in rods, so there’s no way to compare the wavelengths of different photons. We can see, but only based on different intensities of dim light, so everything looks just like different shades of grey.

Of course all of that only applies to dim light at night—as bright as moonlight, for example. We can still see something like a neon sign at night in color, as long as our eyes receive a high enough density of photons to produce a response in our cone photoreceptors, so different wavelengths of light can be distinguished.

© Joel Benington, 2012.

All images come from Wikimedia Commons, and are used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.

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Written by Joel Benington

July 5, 2012 at 11:28 pm

Posted in biology, science

Tagged with , ,