But if you place a clear container filled with hydrogen gas between the flashlight and the prism, gaps appear in the smooth rainbow of colors, places where the light literally goes missing. The dark absorption lines of a star at rest (left) get shifted towards red if the star is moving away from Earth (right)
A waves frequency (in Hertz) is how many crests pass by a point per second. easily confused with period, which is the amount of time it takes for a full wave to pass by a certain point
