Which of the following emits light characterized by a line spectrum?

• A. A hot, dense solid metal
• B. A heated hydrogen gas in a discharge tube
• C. A glowing filament in an incandescent bulb
• D. Sunlight passing through Earth's atmosphere

Answer: (B)

Verified step by step guidance

1. Understand the difference between continuous and line spectra: A continuous spectrum contains all wavelengths of light without interruption, while a line spectrum consists of discrete wavelengths emitted by atoms or molecules transitioning between energy levels.
2. Recognize that hot, dense solids (like a metal or a filament) emit a continuous spectrum because their atoms are closely packed and their energy levels overlap, producing a broad range of wavelengths.
3. Identify that a heated hydrogen gas in a discharge tube is a low-density gas where electrons in hydrogen atoms get excited and then fall back to lower energy levels, emitting light at specific wavelengths, which produces a line spectrum.
4. Note that sunlight passing through Earth's atmosphere is essentially continuous light from the sun, possibly with some absorption lines (Fraunhofer lines), but the source itself is a continuous spectrum emitter.
5. Conclude that only the heated hydrogen gas in a discharge tube emits light characterized by a line spectrum due to atomic electronic transitions producing discrete emission lines.