A researcher investigates how the temperature of a gas affects the appearance of its spectrum. The same atomic gas is placed in two different regions along a single optical path.
• Region X contains the gas heated to 7000 K at very low pressure so that it emits radiation without becoming optically thick.
• Region Y contains the same gas at 2000 K, also at low pressure, and is positioned downstream so that any light from Region X must pass through Region Y before reaching a spectrometer.
Assume that the gas in Region Y remains predominantly in its ground state.
(a) Describe qualitatively the spectrum produced when only Region X is present in the optical path.
(b) Describe qualitatively the spectrum produced when light from Region X subsequently passes through Region Y before entering the spectrometer.
(c) Explain, using atomic energy level populations and photon–atom interactions, why the spectral features change when Region Y is added.
(d) The researcher gradually raises the temperature of Region Y while keeping Region X unchanged. Predict and explain how both the emission and absorption features in the combined spectrum will evolve as the temperature of Region Y approaches that of Region X.