A laboratory spectroscope is used to examine light from two separate experimental…
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
A laboratory spectroscope is used to examine light from two separate experimental setups:
• Setup I: A glass tube containing low-pressure sodium vapor is connected to a discharge source. Bright yellow lines at a wavelength of 589 nm are observed on an otherwise dark background.
• Setup II: A broad-spectrum (white-light) lamp shines through an identical, but unenergized, sodium vapor tube before the light enters the same spectroscope. A pair of dark lines at 589 nm now appear superimposed on the continuous spectrum.
Answer the following questions about these observations in sufficient detail to demonstrate your understanding of the underlying physical processes.
(a) Describe the microscopic process occurring in the sodium atoms inside the energized tube of Setup I that produces the bright yellow emission lines.
(b) Describe the microscopic process occurring in the unenergized tube of Setup II that produces the dark absorption lines.
(c) Explain why the wavelengths of the dark lines in Setup II are identical to the wavelengths of the bright lines in Setup I, even though one case involves emission and the other involves absorption.
(d) Sodium atoms in Setup II can also emit 589 nm photons after absorbing them, yet the spectroscope records a net decrease in light at those wavelengths. Explain why the re-emitted photons do not eliminate the observed dark lines.