Violet light is slowed in a glass of water significantly more than red light.As a result shorter visual waves refract more than do longer ones.Stand outdoors to see and feel the radiation pouring from the Sun.Most of the energy of the Universe is transported in this way, by radiation.The violet limit therefore falls at 4000 A and the red limit near 7000 A or a bit longer.Infrared runs from the red limit to about 0.1 millimeter, and the radio to as long as you wish, even to kilometers."Spectra" is embedded with links that will take you back to the appropriate parts of the above two sites. The classic colors red, orange, yellow, green, blue, and violet connect in a seemingly infinite number of shades, one blending smoothly into the next.
If with superhuman eyes you could see beyond red, you would encounter the "infrared" -- felt as heat on the skin -- which would merge gradually into the familiar "radio" portion of the spectrum.
They are unified by thinking of them as "electromagnetic waves," waves of alternating strength in electric and magnetic fields that all move through space at the "speed of light" (called "c") of 300,000 kilometers per second (186,000 miles per second), eight times around the Earth in the tick of a clock.
The differing kinds of radiation simply have different wavelengths, that is, the separations between crests in two successive waves.
This site, closely coupled to The Natures of the Stars and The Hertzsprung- Russell (HR) Diagram, provides an introduction to the spectra of stars and allied celestial objects.
Here we examine the principal way in which astronomers have learned so much about the stars. Pass sunlight through a triangular prism or bounce it off the finely grooved surface of a compact audio disk and see it break merrily into a band of pure sparkling color, its "spectrum," familiar in the colors of a rainbow, in light glittering from newly fallen snow, in the rings and haloes around a partly- clouded Sun and Moon, in the flash of a cut diamond, and in so many other facets of nature.
Refracted light is therefore "dispersed" or spread out into its spectrum, creating a rainbow -- or the spectrum of a star.