What is the most common visible light source?

• A. Tungsten
• B. Hollow cathode
• C. Mercury vapor
• D. Xenon arc

Answer: (A)

For visible-light photometry, you want a light source that provides a smooth, continuous spectrum across the visible range. A tungsten filament lamp delivers a near-continuous spectrum (white light) that is stable, inexpensive, and easy to power, which is why it is the standard choice in most labs. The other options produce light at specific wavelengths (hollow cathode), strong discrete lines (mercury vapor), or have different practical characteristics (xenon arc) that aren’t as suitable for routine visible measurements.