Voltages are also found in bodies of living organisms. Voltage signals transmitted through nerves to our muscles make them contract. These voltages are very small, in the order of millivolts. Recently researchers found a way to artificially generate these signals and send them to muscles of disabled people who are unable to walk. Examples of very weak signals include those generated by light from distant stars and galaxies.
Astronomers use very large telescopes to gather as much of light as they can.
Electric signals produced by this weak light may be in the range of nV. Astronomers have a way to accumulate these signals by collecting for a very long period of time, and so the present day astronomical images are usually a result of long-time accumulation. For example the Deep Field Image of the most distant galaxies photographed by the Hubble telescope required 276 exposures taken on 10 consecutive days. Now we turn to frequency ranges. But as we concentrate on electronics only, we will loosely divide up the electronics frequency range into the audio range, video range, RF range and microwave range. Audio range refers to the frequencies of sound that we can hear, and in practical terms it means approximately 20 Hz to 20 000Hz.