Tuesday, 24 January 2017

Introducing waves

The idea of waves plays a big part in quantum theory. We'll see later that fundamental particles seem sometimes to behave like waves instead of points of matter. Now we need to get more familiar with the notion of a wave.
Roughly speaking, a wave is a spread-out vibration or periodic change in some quantity. Imagine a guitar string. If you pluck it, the string vibrates from side to side. The movement of the string is a type of wave.
The amount by which the string moves to the side is called the 'amplitude' of the vibration, and it varies along the length of the string. On a guitar string that has been carefully plucked at the middle, the amplitude is greatest at the middle and gradually gets smaller towards each end. The amplitude has to be zero at the ends because they are fixed in position.
The frequency of a vibration is the number of times it vibrates in a second. We'll see later that a typical wave is actually a combination of vibrating movements with different frequencies, but usually there is one vibration in a wave that is distinctly bigger than the others, so we can loosely talk about  the frequency of that bigger vibration as being the frequency of the wave.
Some waves- like the waving of a guitar string that is carefully plucked in the middle- happen in a fixed position. They are called standing waves. Other waves- like the waves that move across the surface of the sea- are called travelling waves.(With the right equipment you can set-up standing waves on water).
Sound is a vibration. When you listen to a radio, the loudspeaker in the radio vibrates, and that makes the air in front of the speaker vibrate too. The vibrations spread through the air (it is a travelling wave) and enters your ear, where it makes the tiny parts in your ear vibrate. Your brain interprets the vibrating of your ear as sound.
When you speak your vocal chords vibrate and cause the air coming out of your mouth to vibrate too. For a typical human voice the frequency of the vibration ranges from around 200 vibrations a second to a few thousand. The faster the vibration the 'higher' the sound appears to the human ear. 

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