What do spectrographs do to light entering it?
A spectrograph separates the light into lines of colored light each with its own distinct wavelength.
White light is actually a combination of all the wavelengths of light.
A prism will separated white light into the 7 colors of the rainbow that we are familiar with.
The light given off when a tube of Helium is excited by an electric current is a combination of all the wavelengths of light given off by the changes in the quantum energy levels of the Helium atom. These quantum energy levels are unique to Helium
The spectrograph will separate the light from each quantum energy change or jump . The spectrograph will then give a picture of all the energy level changes in Helium and the wavelengths of light associated with those energy levels.
The unique spectrograph image of Helium allowed the element of Helium to be identified in the light from the sun, before it was discovered in the atmosphere of the earth. (hence the name Helium meaning from the sun)