Passage V

Ultraviolet (UV) light is harmful to skin. A sunscreen applied to the skin lessens the skin’s exposure to the harmful light, such as through absorption of the light. A sunscreen’s sun protection factor (SPF) is a measure of its ability to lessen exposure.

Students did 2 experiments to measure the absorbance of UV light by various solutions at 25°C, each solution containing a single sunscreen. To make their measurements, the students used a spectrophotometer (see diagram).

In each trial of the experiments, Steps 1−4 were performed:

1. A solution having a particular sunscreen concentration was prepared.

2. Four mL of the solution was poured into a clear plastic vial, and the vial was placed in the spectrophotometer.

3. A beam of UV light was directed through the solution.

4. The absorbance was measured as the wavelength of the UV light was varied from 290 nanometers (nm; 1 nm = 10−9 m) through 400 nm. (Note: UVB light has wavelengths from 290 nm to 320 nm; UVA light has wavelengths from 320 nm to 400 nm.)

Experiment 1

Five sunscreens were tested. Each sunscreen had a different SPF, but the concentration of each sunscreen in solution was the same (0.10 g/L). The results are shown in Figure 1.

Experiment 2

Three sunscreens—the SPF 8 sunscreen, the SPF 15 sunscreen, and the SPF 50 sunscreen—were tested, each at 3 different concentrations in solution (0.025 g/L, 0.050 g/L, and 0.20 g/L). Table 1 shows the maximum absorbance by each solution.

Figure 1 and Table 1 adapted from J. R. Abney and B. A. Scalettar, “Saving Your Students’ Skin. Undergraduate Experiments That Probe UV Protection by Sunscreens and Sunglasses.” ©1998 by the Division of Chemical Education, Inc., American Chemical Society.