A scientist is investigating the emission spectra of two different gases. Gas X produces a spectrum with lines in the visible and ultraviolet regions. Gas Y produces a spectrum with lines only in the infrared and visible regions. Explain how line emission spectra are produced and why the photons emitted by Gas X include higher energy photons than those emitted by Gas Y.

Eduqas A-Level Physics — 2.7 Photons · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When electrons in excited atoms fall back to lower energy levels, they release energy as photons. Each element produces a unique pattern of spectral lines.

Model answer (5 marks)

Electrons in atoms occupy discrete energy levels.

When an electron absorbs energy it is promoted to a higher level (excited state).

The electron then relaxes to a lower level, releasing the excess energy as a photon.

The photon energy equals the energy difference between the two levels: (E=h

Mark scheme (5 marks)

  1. Electrons in atoms exist at specific/discrete energy levels
  2. When an electron absorbs energy it moves to a higher energy level (becomes excited)
  3. When the electron falls back to a lower energy level it releases a photon
  4. The energy of the photon equals the difference in energy between the two levels
  5. Gas X emits higher energy photons because its electrons fall between energy levels with a larger energy difference / ultraviolet photons have higher frequency so higher energy than infrared photons

Key terms in this question

photon

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