Under which conditions is a dark-line (absorption line) spectrum produced?

• A. When a continuous spectrum of light passes through a cooler, low-density gas
• B. When a hot, low-density gas emits light without any background source
• C. When a hot, dense solid or liquid emits light directly
• D. When a continuous spectrum of light passes through a hotter, high-density gas

Answer: (A)

Verified step by step guidance

1. Understand that a dark-line or absorption line spectrum occurs when light from a continuous source passes through a cooler, low-density gas.
2. Recall that the continuous spectrum is produced by a hot, dense object (like a solid or dense gas) emitting light at all wavelengths.
3. Recognize that when this continuous light passes through a cooler gas, atoms in the gas absorb specific wavelengths corresponding to their electronic transitions.
4. These absorbed wavelengths appear as dark lines (absorption lines) in the otherwise continuous spectrum because those photons are removed from the beam.
5. Therefore, the condition for a dark-line spectrum is: a continuous spectrum of light passing through a cooler, low-density gas that absorbs certain wavelengths.