Sunscreens

This is a dry lab whose purpose is to illustrate the concepts of absorption spectra and UV-A, -B, and -C radiation. Students are given transmission spectra of PABA, oxybenzone, octyl (actually 2-ethylhexyl) salicylate, and a broad-spectrum commercial sunscreen ("NO-AD" SPF 45), with wavelengths ranging from 240 to 520 nm. They are told that the region above 520 nm isn't shown because there is no absorption there. They are asked to mark off the UV-A, -B, and -C regions, shade the areas above the graph's curve, and estimate the percentage of UV-A and UV-B that is absorbed (the area of their shaded regions). They are then asked whether they would expect any of the samples I had used to be colored (no, because none absorb in the visible range) and whether holes in the UV-C part of the spectrum (notable with octyl salicylate) are of concern in a commercial product (no, because UV-C never reaches the ground).

Temporary instrumental difficulties necessitated that I obtain spectra manually. The results, in 5-nm increments from 240 to 400, are available from this link. The link also provides the data as an Excel spreadsheet to facilitate graphing.

Revised 9/1/06