Which light source covers the spectrum from ultraviolet to visible?

• A. Hydrogen/Deuterium
• B. Sodium Vapour/Mercury
• C. Tungsten/Laser
• D. Light Emitting Diode

Answer: (B)

Emission spectra determine whether a light source can cover ultraviolet to visible wavelengths. Mercury lamps emit multiple spectral lines that appear in both the ultraviolet and visible regions, providing light in UV and several visible wavelengths. A sodium vapour lamp adds a strong, characteristic line in the visible yellow region around 589 nm. Using both together gives emission that spans from UV through the visible range, making this combination capable of covering the entire UV–visible spectrum.

In contrast, a tungsten filament produces a continuous spectrum mainly in the visible and near-infrared with little UV; LEDs emit light in narrow spectral bands rather than across the whole range; and a hydrogen (or deuterium) lamp yields lines in both UV and visible but not a continuous coverage across the spectrum.