In plotting the transmission experiment graph, which quantity is represented on the y-axis?

• A. Wavelength
• B. Relative absorption
• C. Photon count
• D. Transmission percentage

Answer: (B)

When you plot a transmission experiment, you’re looking at how the sample takes up light across different wavelengths. The quantity you put on the vertical axis is how much light is absorbed relative to what was sent in, i.e., the relative absorption (often expressed as absorbance or as a fraction of I0 that is absorbed). This focus on absorption makes the spectrum reveal the wavelengths where the sample has strong transitions, and it ties directly to how the material interacts with light, independent of fluctuations in the light source. Raw photon counts can vary with the lamp and over time, so they’re less informative for comparing across wavelengths. Transmission percentage sits on the opposite side of the same relationship (absorption is related by T = 1 − absorption), but in spectroscopy the standard way to interpret and compare spectra is through relative absorption, because it aligns with how absorbance scales with concentration and path length via the Beer-Lambert law. The wavelength, meanwhile, is the variable on the horizontal axis, not the vertical one.