You’re unlikely to have heard of the Kramers-Kronig relations in the physics of light and other electromagnetic radiation. These interesting laws of physics relate how strongly the absorption of light at certain wavelengths (colors, essentially) ties to the refraction or bending ability of light at other wavelengths. There are real-life applications, including adding dyes to living tissue to see reasonably deeply inside.
Our tissues don’t let light get deep inside without it scattering in many directions, making imaging impossible more than a few cell depths in. The scattering arises because watery parts of the cell bend light differently from the fats and proteins. Past attempts so see deep inside relied on several techniques, including replacing water in the tissue with glycerol (glycerine) – not a good thing for keeping tissues alive.
Zihao Ou and 20 colleagues at Stanford University realized that large concentrations of the yellow food dye tartrazine (absorbing blue light) would make the watery parts more refractive in the red, matching pretty well the refractive index of the fats and proteins in the cell. Result: very little scattering. The researchers applied strong concentrations of tartrazine to the skin surface of mice to let them see internal organs! Don’t try this at home; though tartrazine in concentrations used in food are safe, the safety of high concentrations needs to be checked.
This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org.
Source: Nature 14 Nov. 24, pp. 359 ff.; Physics Today Nov. 24; Scientific American Dec. 24
