A very common question you may have asked or you might hear from a child is: “Why is the sky blue?” An excellent question that goes right along with: “Why are sunsets red?” This answer to this mystery was explained by British physicist Lord Rayleigh.
Light scatters (think of it as bouncing) off the molecules in the atmosphere. In space, there is no atmosphere, so nothing to bounce off of, so the sky looks black.
But when sunlight hits the atmosphere, we can see any light that bounces toward our eyes. It turns out from Rayleigh’s theory that shorter wavelength light, such as violet and blue, scatters much more efficiently than other colors.
The scattered light is made of all the colors of the rainbow, but more of it is violet and blue. Green is scattered a bit less, followed by yellow and orange, and, lastly, red. The resulting mix we see is what we would call sky blue.
In the western sky at sunset, what we see are the leftovers. Going through more atmosphere, even more blue gets scattered in other directions, leaving the red end of the spectrum as what we see shining toward us.
Beside explaining why the sky is blue, Lord Rayleigh had many contributions to the field of physics. He won the Nobel Prize in Physics in 1904 for the discovery of the noble gas argon.
In 2014, the Nobel Prize in Physics was awarded to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources.”
LED technology lights are more energy efficient and should have a much longer life expectancy, especially when compared with conventional incandescent bulbs. Recently, LED streetlights have been appearing in Durango as part of new construction, and as replacements when older lights fail.
But here is the concern. The newer lights emit more blue light than the currently prevalent sodium street lights. And if the LED lights are cheaper to operate, there is the temptation to install them in “extra” locations that would not have been thought worthwhile last century.
After the Nobel Prize announcement, the International Dark Sky Association made a statement urging the responsible use of LED lights, especially at night. The group points out that historically, whenever lighting technology has become cheaper, more lights were installed for outside use. Several other concerns about blue-rich light are mentioned in that and previous articles.
Blue-rich light directly contributes to more atmospheric scattering. If you are out in the county, you can look back and see a small dome of light in the sky over Durango, and a much larger dome over Farmington. Generally, the bigger the city, the bigger the associated light dome. And now, the overall blueness of the light emitted will directly contribute to the size of these light domes, and decrease the number of stars that can be seen.
Last year, the new LED lights installed at the Old Fort Lewis campus were significantly yellower than the lights they replaced. Yes, the color rendering is not as good as the white, but the view from the observatory has been improved considerably.
I am all for saving money by replacing existing lights, but the savings should be pocketed or spent on other projects and not used to install more outdoor lights.
Get the blankets out. Orion and his winter friends are heading this way. And keep the lights off when you go back inside.
firstname.lastname@example.org. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.