# Short, shorter, shortest

As mentioned earlier, the X-rays that AXAF will observe are photons, just as radio waves, visible light, and gamma rays are made of photons, but all with different energies (cosmic rays are different; they are high-energy, high-speed bits of atoms or sometimes even entire atomic nuclei).

Scientists use three different yardsticks for measuring photons: frequency, wavelength, and energy. Which one is used depends on which will give a small number that is easy to use. Each can be converted to another with the right math since each is related to the speed of light or other physical constants.

Frequency is used to measure radio waves in cycles per second or Hertz (after the physicist who invented it). The FM band on your radio covers the range from 88 to 105 million cycles per second (or, 80 to 105 megahertz or MHz).

Wavelength also is used to measure radio waves. An FM station broadcasting at 88 MHz puts out radio waves with a wavelength of about 3.5 meters (divide the speed of light, 300,000 km/sec by the wavelength and you get the frequency).

As the wavelength grows shorter we move on to millimeter waves, infrared (heat) radiation, and visible light, and so switch to a different unit of measure, the nanometer, or billionth of a meter. Visible light spans a narrow range of wavelengths, from about red at 700 nm to violet at 300 nm.

As we move to X-rays, we switch yardsticks again and use the energy carried by the photons. X-rays have wavelengths of 100 to 10 nm (some scientists prefer to use Angstroms; 10 nm = 1 A). Here we use the energy as measured in electron volts or eV. Visible light carries about 1 to 2 eV of energy. X-rays that AXAF will study carry 100 to 10,000 eV (or 0.1 to 10 keV, kiloelectron volts).

So, if X-rays carry so much energy, why don't we use them to cook food? Zap dinner at 10 keV for 10 seconds instead of 10 minutes in the microwave, and it cooks evenly throughout the meal.

That won't work because of how matter and energy interact. Radio energy and light act on the outer electrons in matter (the microwave oven is tuned to the a frequency that will make hydrogen atoms vibrate - so the water in food gets hot). X-rays will act on atoms differently. They have so much energy that most pass through matter; that's why X-ray radiation shielding has to be so thick.