20 rules for celestial dating
Shorter than violet you would see the ultraviolet, that which gives us tans and sunburns, and then you would encounter much more dangerous X-ray radiation and finally deadly gamma rays.
Except for the energy they carry, all portions of the spectrum -- ordinary light, infrared, radio, ultraviolet -- are fundamentally the same.
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.
Though light and its partners can act like waves, at the same time they can act like a stream of particles.
The oldest form of the device was visual (a spectro SCOPE), and consisted of little more than a prism in a tube fixed to the end of the telescope, the refracted light focused by an observer's eyepiece.
Visual radiation is in the middle, with wavelengths that extend from 0.00004 centimeters for violet light to about 0.00007 centimeters for extreme red.
These wavelengths are so short that astronomers use a small unit of distance, the "Angstrom" (A), which is 0.00000001 centimeters long.
By the turn of the 20th century, spectra were being recorded photographically.
In the middle of the century, prisms were replaced by "diffraction gratings," finely ruled surfaces that produce spectra by the interference of light waves.