What are the two lights described in fluorometers?

• A. Excitation light (short wavelength, high energy) and emission light (long wavelength, low energy)
• B. Absorption light and emission light
• C. Scattered light and transmitted light
• D. Primary light and secondary light

Answer: (A)

In fluorometry, you work with two distinct light components: the light used to excite the molecules and the light they emit after being excited. The excitation light has a shorter wavelength and higher energy, chosen to be efficiently absorbed by the fluorophore. Once the molecule absorbs energy, it quickly relaxes and releases a photon as it returns to the ground state, producing emission light with a longer wavelength and lower energy. The difference in energy—and thus wavelength—is called the Stokes shift. The instrument is designed to separate and detect this emission light while filtering out the excitation light, giving a signal specific to fluorescence.

Why the other pairings don’t fit: absorption light isn’t the emitted signal measured in fluorescence, and scattered or transmitted light aren’t the characteristic “two lights” used to describe excitation and emission. Primary vs secondary light isn’t the standard framework for fluorometers.