Pro-Human Extremist

Extremism in the defense of humanity is no vice

Posts Tagged ‘light

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.


Written by Joel Benington

July 5, 2012 at 11:28 pm

Posted in biology, science

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Why is the sky blue?

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I wrote my post on ten questions about the physical world on a whim, because it had been almost a month since my previous post, and I was backed up with exams at the end of our fall semester, and wanted something I could get out quick. But having done so, I figure I really ought to provide quick explanations to save people the trouble of hunting them up themselves. I’m also intrigued by the challenge of writing brief but reasonably complete answers to these questions.

Why is the sky blue? The reason it’s not black is because light reflects off the molecules of the atmosphere. Without that atmosphere, the only light we’d see in the sky would be what comes straight from the sun, stars, and planets, and the sunlight reflecting off the moon. The rest of the sky would be as black as at night, even though everything around us on earth would be lit up as bright as day.

But thankfully there is an atmosphere, and some of the sunlight passing through the layer of nitrogen and oxygen gas surrounding the earth hits the molecules of that gas and reflects off at an angle. We see the light that reflects at just the right angle to strike the light-sensing cells in our eyes. Enough light is reflected to light up every corner of the sky—not nearly as bright as the sun itself, but enough to make it appear far from black.

Why is it blue? The different colors we see are a result of how much energy is in different photons of light. Higher-energy photons look blue to us, lower-energy photons look red, and the colors of the rainbow between red and blue represent a range of energies from lower to higher.

Clouds look white to us because the tiny water droplets in them reflect all photons equally, so the light coming to us from clouds contains all the colors of the rainbow, and all colors taken together appear white. If the molecules of the atmosphere also reflected all photons equally, the whole sky would look white to us. But since higher-energy photons are more likely to reflect off the molecules of the atmosphere than lower-energy photons are, the sky looks blue.

Why do clouds look yellow and red at sunset? When the sun is close to the horizon just before it sets, the sun’s light is passing sideways through the atmosphere, which means it has to pass a greater distance through the atmosphere before it gets to our eyes. The atmosphere surrounding the earth is extremely thin relative to the diameter of the planet, so when the sun is right on the horizon, its light actually does pass through much more atmosphere than when it shines down from overhead.

As light passes through more atmosphere, more of the higher-energy photons are reflected in other directions, so the light that is left has a relatively greater number of lower-energy photons. The round orb of the sun that we see is made up of the photons that have come straight from the sun to our eyes. It turns more yellow and then orange as it sets because more of the higher-energy photons have been stripped out of the beam of light by reflecting off of molecules in the atmosphere. And since clouds reflect photons of all energies about equally, some of the yellow-orange light that strikes them reflects in our direction, making them look yellow-orange. They turn from yellow to orange to red as the sunset advances because more and more of the higher-energy photons are stripped out of the beam of light as that light passes more sideways through the atmosphere, as the sun sinks lower in the sky.

© Joel Benington, 2012.

Written by Joel Benington

January 3, 2012 at 4:15 pm

Posted in science

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How well do you understand the physical world?

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Some time ago, I realized that I had never really wondered about the physical causes of many little things that were happening around me. We all live in a physical universe, and yet most of us take its operations for granted. We may have the comfortable conviction that there’s a scientific explanation for everything we see, and so we may consider ourselves rationalists. But if we don’t ourselves know the explanations for many of the events of our daily lives, then practically speaking it’s like we are surrounded by magic and mystery.

Over the years, I’ve assembled a list of some strikingly common and obvious phenomena whose causes most people don’t actually know—not because they’re unable to understand, because the explanations are usually not so very complex, but merely because they’ve never bothered to ask.

Here are a dozen of my favorites. How many of them can you yourself explain? I used to make the mistake of cheerfully confronting friends and acquaintances with these questions, but I stopped because I found it usually annoyed the hell out of them. Even though my point was that I had myself spent most of my life blissfully ignorant of the explanations for some of these phenomena, bringing them up in conversation inevitably gave me a know-it-all air, I think. Hopefully blogging about them won’t have the same effect.

1)               Why is the sky blue (and why do clouds look red and yellow at sunset)?

2)               Why does the sun emit light?

3)               Why do things look black and white in moonlight?

4)               Why does water put out fires?

5)               Why do wool and down keep you so warm?

6)               Why is it colder in winter and warmer in summer?

7)               Why does lightning produce the sound we call thunder?

8)               Why do sharp knives cut better than dull ones?

9)               Why does a magnifying glass make things look bigger?

10)          What causes rainbows?

11)          Why can’t you see through milk but you can see through water and vegetable oil?

12)          Why does really hot water make glass crack (and why doesn’t Pyrex crack)?

© Joel Benington, 2011.

Written by Joel Benington

December 4, 2011 at 8:36 pm