general chemistry

Alexandrite Effect: Not All White Light is Created Equal

Alexandrite is a gem that exhibits an amazing property. It appears red in incandescent light and green in sunlight. Incandescent light and sunlight both appear white when we look at them but, as Alexandrite demonstrates, not all white light is the same. Differences in white light sources can have a profound effect on how we perceive an object’s color. The Alexandrite Effect is a perfect example.

Image 1-compared

Blue, green, and red light are defined by single, specific wavelengths of the electromagnetic spectrum (~470 nm, ~540 nm, and ~650 nm, respectively). In contrast, white light is not a single wavelength. It is the sum of multiple, distinct portions of the visible spectrum. Just as many different numbers can be added together to reach 100 (50 + 50, 33 + 67, 99 + 1, etc.) there are many ways to add colors of “pure” light to make white light.

One of the most common man-made sources of white light is black-body radiation. Metallurgist produce white light via black-body radiation when they heat metal in a furnace.  Similarly, incandescent bulbs generate their glow by passing current across a metal element until it heats up and radiates.  Yet, while black-body radiation is an effective means of producing white light, it is very energetically inefficient (most of the energy input is used to produce infrared light/heat). A much more efficient means of creating white light is to combine a few specific wavelengths of the visible spectrum. The color combinations that produce white light are depicted in the CIE color diagram below.

Image 2 CIE

The colors along the rounded edge of the shape (everything but the bottom, straight edge) can be thought of as “pure” because they’re defined by a specific wavelength of light between 380 and 700 nm.  All colors inside the border as well as along the bottom, straight edge are created when two or more “pure” colors are combined. White light is at the “center” of the CIE diagram (x = 0.33 and y = 0.33).

I regularly referenced this diagram while researching organic light emitting diodes (OLEDs) because molecules emit specific wavelengths of light and are not broad emitters (like heated metal). To make an OLED TV that displays most CIE colors, including white, manufactures incorporate blue (x = 0.1666, y = 0.0089), green (x = 0.2738, y = 0.7174) and red (x = 0.7347, y = 0.2653) emitting molecules in the screen design.  To make an OLED screen appear yellow, both the red and the green molecules must be electronically excited and emit at the same time. The resulting color is entirely dependent on the proportion of red and green molecules excited. Exciting more green than red molecules makes the screen appear greenish-yellow. Exciting more red than green molecules makes it appear reddish-orange. If we want the screen to appear yellow, then the intensities of the emitting red and green molecules must be balanced. These “summed” emission can be depicted by drawing a straight line between the red and green points on the CIE diagram (image below).

Image 3-Yellow CIE

Similar strategies are used to generate different types of white emitting OLEDs. Every day, ambient white light is sometimes created by summing the emissions of blue and yellow emitting molecules (image below left). White pixels on OLED TVs are created by summing red, green and blue emitters (image below right).

Image 4- white light cie

We perceive any light source emitting these two color combinations as white. However, illuminating an object, like alexandrite, under these various white light sources can uncover really interesting color chemistry.

Chrysoberyl is an oxide with the formula BeAl2O4 which is typically colorless or yellow because it absorbs little to no visible light. Alexandrite is the rarest and most valuable member of the chrysoberyl family and is formed when some of the Al3+ is replaced by Cr3+, either naturally or intentionally.

The small amount of Cr3+ impurity in Alexandrite (<1 %) is directly responsible for its interesting colors. This coloration can be depicted via the absorption spectrum below.

Image 5- absorbance spectrum

This absorption spectrum is a graphical depiction of the amount of light absorbed/removed/not transmitted (y-axis) versus the wavelength of light (x-axis). Unlike undoped chrysoberyl, Alexandrite has two strong absorption peaks in the visible spectrum at ~400 nm and ~600 nm (for those of you crystal field junkies, the peaks at ~400 nm and ~600 nm are assigned to the 4A2 to 4T1 and the 4A2 to 4T2 transitions of octahedrally coordinated Cr3+). Conversely, it has two low absorptions, or high transmission windows, in the blue-green (470-520 nm) and red (>650 nm) portions of the spectrum.

When Alexandrite is viewed under uniform white light (the sum of ALL visible wavelengths of light) the blue and yellow portions of visible light are absorbed and the blue-green and red portions are not (below left). This gives the gem a purple-grey–the sum of blue-green and red emission–appearance (below right).

Image 6-full white

But, as I said above, not all white light sources are the same. Even though sunlight appears white if you look directly at it (don’t look directly at it!), it actually has a larger contribution from the blue and green portions of the spectrum. Under sunlight, Alexandrite absorbs yellow and blue. Yet, since more green and blue light is transmitted than red light, the gem appears blue-green, as depicted below.

Image 7- sunlight

In contrast, when Alexandrite is placed under incandescent lights or a candle, which have a larger contribution from the red portion of the spectrum, the gem appears red.

Image 8-Candle light

Based on the absorption spectra above, the Alexandrite effect could be greatly amplified if we viewed it under a two-component, white OLED (or a comparable two-color emitter). We could produce a white OLED by combining light from a ~490 nm and a ~590 nm emitter. When viewed under this light source, I’d expect Alexandrite to be a very sharp cyan color because the amber component (590 nm) would be entirely absorbed.

Image 9-cyan from OLED

The relatively narrow emission of molecular species (50-100 nm) would also likely result in a much sharper color for Alexandrite than what is observed in the sun or under incandescent lights. If anyone has an easy way to perform this experiment, I would love to see the result.

That concludes my lengthy but thorough explanation of the interesting color chemistry of Alexandrite, a gem that is sometimes describes as an “emerald by day, ruby by night.” And maybe, experiment pending, this phrase will one day include “cyan by OLED.”


Farrell, E. F.; Newnham, R. E.; The American Mineralogist, 1965, 50, 1972-1981.

Liu, Y.; Shigley, J. E.; Fritsch, E.; Hemphill, S.; Mineralogical Magazine 1995, 59, 111-114.

Five most memorable chemistry papers

Ash over at The Curious Wavefunction has asked for our five most memorable chemistry papers. Here’s mine.

 1) A Specific, Highly Active Malate Dehydrogenase by Redesign of a Lactate Dehydrogenase Framework (Wilks et. al 1988)

If there is a single paper that had the greatest affect on my career decisions then its this one. It quite simply describes how a lactate dehydrogenase was engineered into a malate dehydrogenase. I remember reading it as an undergraduate and marvelling at the fact that mere mortal man had the power to redesign nature’s creations. It drove home the power of molecular biology as well as the wonders of proteins.

I sought out the authors and ended up doing a PhD with one of them.

2 and 3)  A Thermodynamic Scale for the Beta-Sheet Forming Tendencies of the Amino Acids (Smith et. al 1994) and Measurement of the Beta-sheet forming propensities of Amino Acids (Minor & Kim 1994)

These two are both really nice papers that I referred to a lot during my PhD. But the main reason I’m including them in my list is that they are IDENTICAL (within experimental error)! The two groups selected the same protein, made the same mutations, did the same experiments, (reassuringly) got the same results and then published at the same time.

You’ve got to feel sorry for Smith et al. they published in good old Biochemistry, whilst Minor and Kim got a Nature paper. There has got to be a fascinating back story about what happened here, but I’m afraid I have no idea what it is. Maybe I’ll write to the authors and find out.

4) A Day in the Life of Dr K. or How I Learned to Stop Worrying and Love Lysozyme: A Tragedy in Six Acts (Gunnar von Heijne 1999)

Why are papers so often so dry? Here’s the perfect counter to all that dusty language, a review article written in play form and to cap it off diagrams sketched out on napkins. Plus its another career altering paper, it  pushed me towards membrane proteins.  Screen Shot 2013-08-09 at 14.09.09

5) Blackawton bees (Blackawton et al. 2010)

This one is a bit of a cheat, because its very obviously not a chemistry paper. However it is a lesson to us all on how to write a paper. Its clear, jargon free and a joy to read.  Why can’t all papers be this clear? Probably because they are rarely written by primary school children. And don’t forget to check out the crayon diagrams and tables drawn with pencil. 

By August 9, 2013 1 comment general chemistry

Twitter Brain’s Chemistry Novel (and other book) recommendations

I’ve been looking for an easy to read book (fiction or non-fiction) to send out to chemistry students before they arrive at Uni. The plan is to have all our first years read the same book before they arrive. With any luck it will give them something to chat about and give our first few lectures a point of reference.

So I asked the twitter brain for its chemistry book recommendations, and here’s what it came up with.

  1. @Sci_ents @DrRubidium Anyone say Greg Benford’s Timescape? More physics but includes NMR, time travel, eco-disaster, and academics.
  2. @Sci_ents @DrRubidium I can recommend an author… Peter Watts.. his first series is chock full of science goodness including chemistry
  3. @sci_ents we’re partial to this one: Short stories about a deadly assassin who uses a different poison for each kill
  4. @ChemistryWorld @Sci_ents My friend told me to read “The Disappearing Spoon” by Sam Kean. I just checked it out from the library!
  5. @Sci_ents I enjoyed ‘The Girls of Atomic City.” It tells the story of the nuclear bomb development from the “blue collar” people working…
  6. @simonbayly @Sci_ents @ChemistryWorld It was Mr Levi whom inspired me onto the chemical trail at age 14. Highly recommended reading.
  7. @BytesizeScience @Sci_ents Goethe’s “Elective Affinities” is a Classical example, but highly metaphorical. Downhill from there.
  8. @Sci_ents @ChemistryWorld Mr Tompkins by George Gamow
  9. @Sci_ents @ChemistryWorld The Periodic Table by Primo Levi isn’t a novel exactly, but it is one of the best books ever.
  10. @Sci_ents @ChemistryWorld @Sci_ents not sure if this counts but “cat’s cradle” by Vonnegut has some nice ideas
  11. @Sci_ents Interesting physics and chemistry in Reflex by Dick Francis. Not exactly concepts though, more application.
  12. @Sci_ents @ChemistryWorld not really a novel but The Periodic Kingdom by P W Atkins is a great read
  13. @Sci_ents The Documents in the Case, Dorothy L. Sayers. Not much chemistry until the clincher which is chemical concept. (DM for spoiler)
  14. @Sci_ents When I was undergrad, one grad inorg cume at WUSTL included question, “Who killed Missy Moonbeam in The Delta Star?”
  15. @Sci_ents @ChemistryWorld
    Uncle Tungsten: Memories of a Chemical Boyhood by Oliver Sacks
  16. @Sci_ents @ChemistryWorld Susan Gaines’ Carbon Dreams?
  17. @Sci_ents @ChemistryWorld – Napoleon’s Buttons: How 17 Molecules Changed History, Penny Le Couteur
  18. @Sci_ents Not exactly fitting the criteria but Primo Levi’s Periodic Table comes to mind
  19. @Sci_ents Primo Levi’s The Periodic Table some named as best science novel ever
  20. @Sci_ents @ChemistryWorld or cat’s cradle if gravity’s rainbow is too much of a slogger
  21. @Sci_ents Emm short answer no. Long shot- Dune. Spice as a drug, water harvesting and terraforming. Best I can do ad hoc
  22. @Sci_ents A Whiff of Death (I. Asimov) — murder mystery set in Chemistry department… The Delta Star (J. Wambaugh) — similar plot.
  23. @Sci_ents @ChemistryWorld gravity’s rainbow, imopolex g


Did we miss any?

What’s with all the “periodic tables”?

I don’t know about you but I’m sick of all the imitation “periodic tables” doing the rounds. They just don’t make sense. How are periodic tables of muppets, candy or fonts anything but a collection of things that fit into those catergories. There’s rarely any periodicity to any of them, so why  call them a periodic table? Aren’t they just, well … tables? OK there are a few gem; ironically the periodic table of irritational nonsense is quite good and the periodic table of periodic tables deserves a mention. But on the whole aren’t they  just a load of clap trap.

Now if you want a really good varient of the periodic table how about this from science communicator and chemist Jamie Gallagher?  


By June 26, 2013 2 comments fun, general chemistry