Final answer:
To observe a line spectrum, a substance must be excited typically by an electrical current or heat, causing it emit light at specific, quantized energy states unique to each element.
Step-by-step explanation:
In order to observe a discrete (line) spectrum, the substance creating the spectrum must be excited in some manner, such as by passing an electrical current through a gas at low partial pressure or by heating it. Fluorescent light bulbs and neon signs operate on the principle of creating line spectra. Each element exhibits a unique set of lines in its emission spectrum, which are produced due to quantized energy states of the atoms. Seeing only certain lines of light through a prism is indicative of a line spectrum, which is characteristic of individual elements and significantly different from a continuous spectrum that a hot object emits.