Figure A. YES! The sky is blue. Just look up on a sunny day and voila....you have your answer. The sky is blue. But, wait...as a curious scientist should always ask, what is it that causes the sky to be blue? And this is where things get a little tricky. Enjoy!
Physicists tell us the sky is blue because blue light in the sun's rays has a shorter wavelength than red light. But purple light has an even shorter wavelength than blue light, so why is the sky not purple then??
The answer is...the sky is purple too! Here is how it works:
Traditionally we were taught that sunlight is scattered more for shorter wavelengths (purples and blues) than for longer ones (reds and yellows). So when the full spectrum of light from the sun shines onto Earth, it is scattered, or deflected, by molecules in the atmosphere — primarily nitrogen and oxygen. Shorter wavelengths are scattered more than longer wavelengths. So when we look into the sky, we see the wavelengths that are bent the most. The light of day is actually a complex spectrum of many different wavelengths, but it is dominated by light with wavelengths between 400 nanometers (purple) and 450 nanometers (blue). So those are the two colors we should see in the sky. But we don't. And this is where things get personal...for humans.
The human eye is sensitive to light between roughly 380 and 740 nanometers. On a typical retina, there are 10 million rods for sensing low light levels and 5 million cones for detecting color.
Each cone contains pigments that restrict the range of wavelengths that the cone responds to. There are three varieties of cones for long, medium and short wavelengths. All three cone types are needed in order to see color correctly. The peak response for the long cones is at 570 nanometers (yellow), medium at 543 nanometers (green), and short at 442 nanometers (between violet and blue). But the three cones are sensitive over broad, overlapping wavelength ranges, which means two different spectra can cause the same response in a set of various cones.
The Rubik's Cube illusion (Figure A) is a great example of how our eyes can be tricked by these overlapping wavelength's . Believe it or not, squares A, B, and C are all the same color. The shared color cones in our eyes send signals to our brain based on all the wavelengths that they receive. The cones cannot selectively detect a specific color (from a single square) so the mixed context of all the colors striking the cones creates this remarkable illusion.
The same process that makes the squares look like different colors on the Rubik’s Cube is also happening in the sky. But in this case, the sky's combination of purple and blue elicits the same cone response as pure blue plus white light, which is an equal mixture of all the colors.
The human eye can't tell the difference between that complex spectrum and one that is a mixture of pure blue and white. Our color cones register it as the exact same thing. So the sky, to us, is BLUE!
However, in other animals, the color of the sky looks very different. Besides humans and primates, most animals only have two types of cones. Which means they are dichromatic (vs trichromatic like humans). This means they see the world in a some surprisingly different ways.
So the next time you look up, appreciate that what you see as blue, other creatures see as deep purple. The doors of perception are open to all, but what is seen within is governed by the evolved qualities through which the data is gathered and processed.
So to answer the question posited in the title, YES, the sky is blue...to humans. #ScienceRocks!
Physicists tell us the sky is blue because blue light in the sun's rays has a shorter wavelength than red light. But purple light has an even shorter wavelength than blue light, so why is the sky not purple then??
The answer is...the sky is purple too! Here is how it works:
Traditionally we were taught that sunlight is scattered more for shorter wavelengths (purples and blues) than for longer ones (reds and yellows). So when the full spectrum of light from the sun shines onto Earth, it is scattered, or deflected, by molecules in the atmosphere — primarily nitrogen and oxygen. Shorter wavelengths are scattered more than longer wavelengths. So when we look into the sky, we see the wavelengths that are bent the most. The light of day is actually a complex spectrum of many different wavelengths, but it is dominated by light with wavelengths between 400 nanometers (purple) and 450 nanometers (blue). So those are the two colors we should see in the sky. But we don't. And this is where things get personal...for humans.
The human eye is sensitive to light between roughly 380 and 740 nanometers. On a typical retina, there are 10 million rods for sensing low light levels and 5 million cones for detecting color.
Each cone contains pigments that restrict the range of wavelengths that the cone responds to. There are three varieties of cones for long, medium and short wavelengths. All three cone types are needed in order to see color correctly. The peak response for the long cones is at 570 nanometers (yellow), medium at 543 nanometers (green), and short at 442 nanometers (between violet and blue). But the three cones are sensitive over broad, overlapping wavelength ranges, which means two different spectra can cause the same response in a set of various cones.
The Rubik's Cube illusion (Figure A) is a great example of how our eyes can be tricked by these overlapping wavelength's . Believe it or not, squares A, B, and C are all the same color. The shared color cones in our eyes send signals to our brain based on all the wavelengths that they receive. The cones cannot selectively detect a specific color (from a single square) so the mixed context of all the colors striking the cones creates this remarkable illusion.
The same process that makes the squares look like different colors on the Rubik’s Cube is also happening in the sky. But in this case, the sky's combination of purple and blue elicits the same cone response as pure blue plus white light, which is an equal mixture of all the colors.
The human eye can't tell the difference between that complex spectrum and one that is a mixture of pure blue and white. Our color cones register it as the exact same thing. So the sky, to us, is BLUE!
However, in other animals, the color of the sky looks very different. Besides humans and primates, most animals only have two types of cones. Which means they are dichromatic (vs trichromatic like humans). This means they see the world in a some surprisingly different ways.
So the next time you look up, appreciate that what you see as blue, other creatures see as deep purple. The doors of perception are open to all, but what is seen within is governed by the evolved qualities through which the data is gathered and processed.
So to answer the question posited in the title, YES, the sky is blue...to humans. #ScienceRocks!
RSS Feed