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Holidays 2018

December 23, 2018

Red and green are the traditional colors for Christmas decorations. This is the likely consequence of the holiday's historical association with holly (genus ilex), which has green leaves and red berries. The red color is also reinforced by the modern image of Father Christmas (a.k.a., Saint Nicholas and Santa Claus, among other names) wearing a red suit.[1] There's nothing like a good piece of red velvet cake to reinforce holiday redness. Holly was also associated with winter solstice celebrations such as Yule.

Ilex aquifolium (holly) and Santa Claus (jolly)

The holly and the jolly. Left image, Ilex aquifolium, from the Atlas der Alenflora (1882) by Anton Hartinger. Right image, from a 1911 Christmas postcard. Both via Wikimedia Commons

Red is the principal color of many objects of scientific interest, the best examples of these being the many sources of pure and intense red light. The first gas laser, the helium-neon laser, demonstrated in 1962, emits red light at a wavelength of 632.8 nanometers (nm). The first solid-state laser, the ruby laser, demonstrated in 1960, emits a deeper red light at 694.3 nm. Red light-emitting diodes are ubiquitous, and AlGaInP/GaAs LEDs emit at 625 nm.

Spectrum of a red LED

Spectrum of a red light emitting diode (LED).

This particular type of LED is an AlGaInP/GaAs diode.

(Created using Inkscape.)

Planetary astronomers have two huge red objects on their minds, the planet Mars, and Jupiter's Great Red Spot. The reddish color of Mars comes from the iron(III) oxide (Fe2O3) on it's surface. This iron oxide is also known by its mineral name, hematite, derived from the Greek word for blood.

Jupiter's Great Red spot is an anticyclonic storm in the atmosphere of Jupiter that's existed at least since 1830, and it's slightly larger in width than the Earth. The exact cause of its red color is not known, but it might come from a compound of sulfur, or from some complex organic molecules. The spot's redness might also be caused by an allotrope of phosphorus, red phosphorus, that has a deep red color.

There are other examples of redness outside our Solar System, the most important of which is the Doppler redshift of light emitted from the recession of astronomical objects in our expanding universe. The star, Betelgeuse, which is a part of the Orion Constellation, is the ninth-brightest star in the night sky. It's also a red supergiant star of spectral type M. Red supergiants are cool and large. Betelgeuse has a surface temperature of 3590K, compared with our Sun's 5,772K and a radius that's about 900 times the solar radius.

Jupiter's Great Red Spot, the planet Mars, and an image from the Pech Merle cave

Seeing red in three worlds. Left image, portion of Jupiter's Great Red Spot, acquired on July 11,2017 by the Juno spacecraft (NASA/SwRI/MSSS/Gerald Eichstädt/Seán Doran image); middle image, a 1975 Viking 1 orbiter image of Mars, from NASA Website; right image, outline of a human hand, c. 25,000 BC, at the Pech Merle cave, France. This hand image in the Pech Merle cave was created with red ochre, a mixture of a large quantity of hematite with clay. All images from Wikimedia Commons.

Red was the only option for laser pointers for a few years until inexpensive green laser diodes became available. One of my laser engineering colleagues built a green laser pointer for our laboratory before they became a commodity item, but I imagine that the component cost was well over a $100 at that time. Green laser pointers emit at 532 nm, near the peak of the human eye response.

For higher power, green laser light is created by doubling the intense infrared light of a neodymium or chromium-doped host crystal by passage through a nonlinear optical material that acts as a "second-harmonic generator. Common neodymium hosts are yttrium aluminum garnet (Nd:YAG), and yttrium vanadium oxide (Nd:YVO4) Chromium-doped chrysoberyl (Cr:BeAl2O4) also acts as an infrared laser.

It's even possible to generate green light at 543 nm from a helium–neon laser. Argon-ion lasers emit at 514.5 nm and 528.7 nm, and they are often used in laser eye surgery. Copper vapor can be used as a laser at 510.6 nm. There are many minerals colored green, a prime example being emerald, which gets its color from chromium atoms in its beryl, Be3Al2(Si6O18), host crystal. We're all familiar with the green patina on copper, called verdigris, that usually contains copper carbonate (malachite) or copper chloride.

Eye color sensitivity

The color sensitivity of the human eye peaks at green.

This curve was developed by asking study participants to equalize the perceived intensity of various pairs of colored lights, and then doing a mathematical analysis of these data.

(Modified Wikimedia Commons image.)


  1. How Red And Green Became The Colors Of Christmas, NPR All Things Considered, December 20, 2016.

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