Why Is the Sky Blue, and Not Violet? #Shorts
Sunlight looks white, so why is the sky blue? The answer is scattering, and it took two Victorian physicists to work it out.
- Air scatters sunlight in all directions, and it scatters short wavelengths far more than long ones, so blue light reaches your eyes from all over the sky. (NASA Space Place; NOAA NESDIS)
- In 1869, John Tyndall made clouds of very fine particles in a glass tube; seen from the side, the beam looked blue. He linked it to the sky. (Royal Institution; Proc. Royal Society, 1869)
- In 1871, Lord Rayleigh worked out the law: scattering rises with the inverse fourth power of wavelength, so halving the wavelength means sixteen times more scattering. In 1899 he showed the air molecules themselves are enough. (Philosophical Magazine, 1871 and 1899)
- Violet scatters even more, but sunlight carries less of it, some is absorbed high in the atmosphere, and our eyes are less sensitive to it. (NOAA NESDIS; NOAA Storm Prediction Center; UC Riverside Physics FAQ)
- At sunset, light crosses far more air, so most of the blue is scattered out of the beam and reds and oranges remain. (NASA Space Place; NOAA Storm Prediction Center)
Blue skies or red sunsets: which do you prefer?
Sources
- NASA Space Place, "Why Is the Sky Blue?"
- NOAA NESDIS, "Why Is the Sky Blue?"
- NOAA Storm Prediction Center, S. Corfidi, "The Colors of Twilight and Sunset"
- Royal Institution, John Tyndall's blue sky apparatus
- J. Tyndall, "On the blue colour of the sky...," Proc. Royal Society, 1869
- Lord Rayleigh, "On the light from the sky, its polarization and colour," Phil. Mag., 1871
- Lord Rayleigh, "...on the origin of the blue of the sky," Phil. Mag., 1899
- P. Gibbs, Physics FAQ, "Why is the sky blue?" (UC Riverside)
Images
- "ISS-51 Moon, Earth's limb and the thin blue atmosphere", NASA, public domain
- "John Tyndall", painting by John McLure Hamilton, public domain
- "John William Strutt, 3rd Baron Rayleigh", photogravure by Walker & Cockerell, Wellcome Collection, CC BY 4.0
- "Rayleigh sunlight scattering", Robert A. Rohde (SVG by KES47), CC BY-SA 3.0
- "Bering Sea Sunset In Alaska", Allan Shimada, NOAA, public domain
