Which of the following is a line source?

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

Answer: (B)

In spectroscopy, a line source emits light at specific wavelengths because the photons come from particular atomic transitions. A mercury vapor lamp is a classic line source because when mercury atoms in the discharge are excited, they produce bright, discrete emission lines at well-known wavelengths, making it ideal for precise wavelength calibration and line-specific measurements. By contrast, a tungsten filament emits like a hot solid (a blackbody), giving a broad, continuous spectrum; xenon arc and deuterium lamps are also used as broad-spectrum sources for UV/visible regions rather than for discrete lines.