Blue light is given off by hydrogen in a glass tube at low pressures with an electrical current running through it. This light can be passed through a prism to give four distinct bands of color.
When the blue light passes through the prism violet with a wavelength of 410.1 nm, blue-violet with a wavelength of 434.0 nm, blue-green with a wavelength of 486.1 nm and red with a wavelength of 656.2 nm are all seen on a black background. This is called the Balmer series. There are four other series of lines in longer wavelengths (the infrared spectrum) and shorter wavelengths (the ultraviolet spectrum). These are called the Lymann, Paschen, Brackett and Pfund series.