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When dermatologists see the muted indigo discoloration of dermal melanosis or an old tattoo, they knowingly murmur sotto voce, “Ah, witness the Tyndall effect.” But can they explain it?
John Tyndall (1820-1893) was the first to explain why cutaneous hemorrhagic or melanotic lesions, which should appear red-brown or brown-black, respectively, instead appear blue-tinged. Tyndall observed that this phenomenon (Figure) occurred only in nonhomogenous media, in which irregularities cause short-wavelength visible light (ie, at the blue end of the spectrum) to scatter, some reflecting back to the observer. Long-wavelength visible light (at the red end of the spectrum) continues to pass through (and ultimately vanish within) the nonhomogenous medium.
Richey PM, Norton SA. John Tyndall’s Effect on Dermatology. JAMA Dermatol. 2017;153(3):308. doi:10.1001/jamadermatol.2016.0505
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