Which of the following is NOT a continuum light source?

• A. Tungsten filament
• B. Deuterium lamp
• C. Xenon arc lamp
• D. Mercury-sodium vapor lamp

Answer: (D)

Light sources can be described by whether they emit a broad, smooth spectrum (continuum) or light at specific wavelengths (lines). A hot tungsten filament acts like a blackbody, giving a broad, continuous spectrum across the visible (and into nearby infrared). A deuterium lamp provides a continuous spectrum in the ultraviolet region. A xenon arc lamp also emits a wide, smooth spectrum from the ultraviolet through the visible. In contrast, a mercury-sodium vapor lamp emits light at fixed atomic transition wavelengths for mercury and sodium, producing bright spectral lines rather than a smooth continuum. Because of these discrete emission lines, it is not a continuum light source.