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.
Turned to the sky and attached to a detector, the lens becomes an astronomical telescope.
(A curved mirror can create a similar image by reflection.) The speed of an electromagnetic wave in a medium depends on its wavelength.
Though light and its partners can act like waves, at the same time they can act like a stream of particles.
In a crude sense, these particles, called "photons," carry the waves.
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.
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 most familiar is "reflection," in which light is bounced from a surface, the light coming off at the same angle at which it hits, resulting in your undistorted face looking back at you from a mirror. When it passes into a substance, it slows and can be bent, a common phenomenon called "refraction." The effect is easily seen when looking at something through water.