Fluorescein will fluoresce when hit by light at what wavelength?

• A. 450 nm
• B. 515 nm
• C. 600 nm
• D. 700 nm

Answer: (B)

Fluorescein emits light at its characteristic emission wavelength after being excited by blue-green light. Its excitation peak is around 495 nm, and it releases green fluorescence with an emission peak near 515–520 nm. Therefore, the wavelength at which fluorescein fluoresces is about 515 nm, which corresponds to the green emission you detect under standard FITC imaging. The other options lie outside fluorescein’s emission range or reflect excitation wavelengths that don’t produce the typical green fluorescence.