Final answer:
Discrete, clearly defined lines in a spectroscope are indicative of atomic or molecular emission lines, common in fluorescent materials and neon signs, and each element displays a characteristic line spectrum.
Step-by-step explanation:
When observing discrete, clearly defined lines through a spectroscope, a likely source for this spectrum is atomic or molecular emission lines. Unlike an incandescent light bulb, which produces a continuous spectrum, atomic or molecular emission lines appear when a gas at low partial pressure is excited with an electrical current or heat, emitting light only at certain frequencies. This results in a line spectrum with very narrow line widths. Each element and molecule can produce a distinct set of lines, making this spectrum unique to particular substances. Fluorescent materials, such as those in fluorescent light bulbs and neon signs, operate similarly and are known to give off line spectra.