Spectroscopes?

Question

If light were passed through a round hole instead of a thin slit in a spectroscope, how would the spectral "lines" appear? What is the drawback of a hole relative to a thin slit?

Answer 1

In a slit spectroscope, the length of the slit determines the length of the diffraction lines formed by the diffraction grating. The width of the slit determines its resolution and brightness (narrower = higher resolution but dimmer).
Reducing the slit to a pinhole would reduce the diffraction lines to points, making it harder to visually differentiate the colors. Using a larger hole would cause the lines to approach the form of images of the light source (if angularly smaller than the hole) or of the hole itself, and to lose resolution. The illustrations in the ref. will make this understandable.