Which statement defines the band gap?

• A. Energy difference between the valence band and the conduction band.
• B. Color produced by luminescence.
• C. Pattern of dark lines in a spectrum.
• D. Continued emission after stimulation stops.

Answer: (A)

Band gap is the energy difference between the valence band, where electrons are bound, and the conduction band, where electrons are free to move. This gap sets the minimum photon energy required to excite an electron from the valence band to the conduction band, which in turn determines which wavelengths are absorbed and thus the color appearance of the material. In gemstones and other solids, the band gap controls whether the material behaves as an insulator or semiconductor and influences its optical properties.

The other statements describe related phenomena but not the defined band gap: color produced by luminescence refers to light emitted after excitation rather than the energy gap itself; a pattern of dark lines in a spectrum refers to absorption lines in a spectrum; continued emission after stimulation stops describes phosphorescence, not the band gap.