Which of the following best explains why a gas emits light (glows) when an electric current is passed through it in a discharge tube?

• A. The gas molecules undergo combustion, producing visible light.
• B. The gas atoms absorb light from the surroundings and re-emit it.
• C. Electrons in the gas atoms are excited to higher energy levels and emit photons when they return to lower energy levels.
• D. The gas condenses into a liquid, releasing energy as light.

Answer: (C)

Verified step by step guidance

1. Understand that when an electric current passes through a gas in a discharge tube, it provides energy to the gas atoms or molecules.
2. Recognize that this energy excites electrons in the gas atoms, causing them to move from their ground state to higher energy levels (excited states).
3. Recall that excited electrons are unstable and will eventually return to their original, lower energy levels.
4. Know that when electrons return to lower energy levels, they release the excess energy in the form of photons, which are particles of light.
5. Conclude that this emission of photons is what causes the gas to glow or emit light, rather than combustion, absorption and re-emission of external light, or condensation.