Emission spectra are caused by

Analyze the question: It asks what causes emission spectra, meaning the light given out by an excited substance.

Concept: Electrons in an atom occupy fixed energy levels.

• When an electron absorbs energy, it jumps from a lower level to a higher level (excitation). This produces absorption spectra, seen as dark lines on a continuous background.
• An excited electron is unstable, so it falls back from a higher level to a lower level. The energy difference is released as a photon of light:

E = E_higher − E_lower = hf

Each possible transition gives a photon of a particular frequency, which appears as a bright line in the emission spectrum.

Checking the options:

• A. Kinetic energy of moving atoms: this relates to heat and temperature, not to line spectra. Wrong.
• B. Potential energy of an electron: this describes the energy state, not the process that releases light. Wrong.
• C. Transition from a higher to a lower energy level: this is the process that releases a photon. Correct.
• D. Transition from a lower to a higher level: this is absorption, not emission. Wrong.
• E. Excited protons in the nucleus: nuclear transitions produce gamma rays, not the optical emission spectra of atoms. Wrong.

Answer: C

Exam tip: Remember "Emission = Exit of energy = electron falls down." Absorption = electron goes up. If a question mentions bright lines on a dark background, think emission. If it mentions dark lines on a bright background, think absorption.