The longest wavelength and smallest energy electromagnetic wave (or photon) is the radio wave. Since their energies are so small (10^-12eV to 10^-8eV) it is easier to consider them as very- long wavelength waves of radiation. Since all E&M waves move at the speed of light (300,000 km/s or 186,000 mi/s), we can also think of the frequency at which the wave crests pass us. The frequency scales up with the energy, so the rest of the spectrum deals in very high frequencies. At radio energies, the frequencies are a little more manageable, so we use these to denote the different kinds of radio signals.

For instance, the frequency that a popular Valley radio station transmits its signal is 91,500,000 Hz (Hz stands for Hertz, a unit denoting cycles or waves/s). We know this as 91.5 MHz. This is a frequency-modulated signal (FM) in which the frequency of the waves are combined with the amplitude (the information) to create the signal and eliminate noise, the major downfall of amplitude-modulated (AM) signals.

AM signals are transmitted by radio and television stations between 535 kHz and 1605 kHz, but can be picked up by almost anything from standard telephones to orthodontia. All electronic devices emit some radio waves, though usually at very low energies. Any alternating electrical current will generate some E&M radiation, usually in the radio regime.

However, radio waves have been detected in space, much to the advantage of astronomers studying the motions of galaxies and to cosmologists interested in the amount of cold material in the deepest regions of the Universe. Atomic hydrogen emits radio waves when it flips its spin axis, a very rare occurrence. However, the vast abundance of hydrogen in huge clouds throughout space allow us to see how galaxies are moving and rotating.

Not only do we see radio waves from gas clouds, but also from distant galaxies colliding and emitting extremely long-wavelength (millions of miles) radiation.