Absolute and relative dating of rock

In the modern spectrograph, light is sent from the telescope onto a "collimator," a curved mirror that straightens the converging beam.

The collimator sends the beam to a reflecting grating that makes a spectrum, the colored light then focused by a camera onto a detector, usually a "charge-coupled device," or "CCD," that records the spectra digitally.

Ultraviolet runs from 4000 A down to about 100 A, X- rays take over to about 1 A, and these are followed by the gamma rays to no known lower limit.

The named divisions are artificial and serve only to block out large spectral segments.

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.

Infrared runs from the red limit to about 0.1 millimeter, and the radio to as long as you wish, even to kilometers.

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.

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.