How can we measure the absorption probability of different wavelengths for photopigments?

• A. Shine polychromatic light through a box containing rhodopsin and measure the transmitted light with a radiometer
• B. Shine monochromatic light through a box without rhodopsin and measure the transmitted light with a radiometer
• C. Shine monochromatic light through a box containing rhodopsin and measure the reflected light with a radiometer
• D. Shine monochromatic light through a box containing rhodopsin and measure the transmitted light with a radiometer

Answer: (D)

To find how likely a photopigment is to absorb a specific wavelength, test one wavelength at a time and see how much of that light gets through the pigment. Monochromatic light ensures you know exactly which wavelength is being tested, and placing rhodopsin in the path lets the pigment absorb some of that light. By measuring the transmitted light with a radiometer, you directly gauge how much light remains after interaction with the pigment. The portion absorbed is the difference between the incident and transmitted light, giving the absorption probability for that wavelength.

Using polychromatic light would mix many wavelengths, making it impossible to resolve absorption for each individual wavelength. If the pigment weren’t present, there would be nothing for the pigment to absorb, so the measurement wouldn’t reflect absorption by photopigment. Measuring reflected light adds complexity from surface properties and scattering and does not straightforwardly quantify how much of the specific wavelength was absorbed.