It is determined by the nature of the green light. Because lasers create light at almost a single frequency, green laser light would appear as a thin line of pure green. Other sources of "green" light emit light at a variety of frequencies, including yellow and blue, resulting in a strong green band in the center that fades into blue-green and yellow-green at the borders.
For example, here’s a graph of the spectrum of a green LED, showing the color range: Attachment #1
and here’s a graph of the transmission spectra of several standard photographic filters, including green: Attachment #2
Learn more about the color spectrum:
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