The Stimulus for Hearing
We can hear differences in sound because sound waves vary in their frequency and amplitude. The frequency of sound wave is expressed in a unit known as hertz (Hz) (named after it's discoverer, German physicist Heinrich Rudolgh Hertz), which indicates the number of compression and expansion cycles that occur within 1 second. The pitch of a sound--the subjective interpretation of its frequency--depends on its frequency: the higher the frequency, the higher the pitch. Most speech and music are complex mixtures of many frequencies, but a few simple examples will give you an idea of the range of common sounds. The pitch of human voice ranges from 120 Hz to about 1100 Hz, middle C on a piano is 256 Hz, and the highest note on a piano is 4100 Hz. Young adults can hear sounds with frequencies as low as 20 Hz and as high as 20,000 Hz.
The other physical attribute of a sound wave is its amplitude, or intensity, which can be thought of as the height of the wave, where pressure is greatest. Amplitude is usually expressed is decibels (dB). Perceived loudness nearly doubles whenever the physical intensity of sound increases by 10 dB. Among common sounds, a whisper is generally about 20 dB, normal speech seems sixteen times (not three times) louder than a whisper, and a subway train seems eight times louder than normal speech. We experience sounds above 120 dB as painful, which explains why listening to an amplified rock band (130 dB) is unpleasant to many old timers and dangerous to the ears.
The Stimulus for Vision
Light is a kind of energy that is transmitted in a wavelike form. What we actually see as visible light is only a small part of a much larger range of energy called the electromagnetic spectrum. Included in this broad spectrum are radio waves, infrared waves, ultraviolet waves and gamma rays; none of these portions of the spectrum are visible, and their presence can be measured only with special instruments. The energy waves that make up the electromagnetic spectrum are defined in terms of their wavelengths, expressed in units termed nanometers (abbreviated nm; 1 nm equals one billionth of a meter). Visible light ranges from just below 400 to about 780 nm. It is no accident that the human eye evolved sensitivity to this range of wavelengths: longer wavelengths of light would be difficult for the eye to focus.
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