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Newton’s rings
N
Newton’s rings
(See: Interference.) Noise
Any unwanted sound. It may be
a loud sound that disturbs the listener. In radios noise is unwanted electrical signals that accompany the desired signal. They are sometimes called ‘static’ because such noise is often generated
by lightning and other sources
of sparks. The equivalent on a television picture is called ‘snow’. (See also: Pink noise and White noise.)
O
Octave
A musical interval that forms the unit of the modern scale. An octave is a fundamental unit used in nearly all music no matter what culture it comes from. In western music an octave contains eight notes.
The note at the top of an octave has twice the frequency of the note
at the bottom. A one-octave step stretches, for example, from the note C (261 hertz) to its octave C' (522 hertz). The octave from the note A stretches from A (220 hertz) to its octave A' (440 hertz).
A child has a hearing range of more than 10 octaves. A piano keyboard has an octave range of 7.5 octaves.
It is also possible to talk about light in terms of octaves. The range of human sight is much more limited than the range of hearing. It spans just one octave, the lowest frequency light being just half of the highest frequency light that we can see. We call it the spectrum.
Optics
The science of studying light.
Overtone
A musical instrument will produce a fundamental tone, for example, middle C. When this note
 Overtone – A complete wavelength is a fundamental. One-half a wavelength is a first overtone, one-third of a wavelength is a second overtone, and so on.
1 wavelength
is played, a range of overtones will also be produced that are
at frequencies higher than the fundamental tone. They combine with the fundamental tone to give the sound richness.
In most instruments, for example, woodwinds and strings, the overtones are exact multiples, or harmonics, of the fundamental tone. On a piano the overtones are not all exact multiples of
the fundamental. These features are responsible for the special characteristic of piano sound. (See also: Timbre and Wavelength.)
P
Percussion
A range of musical instruments
in which a skin (in the case of instruments like drums) or a metal (as in a xylophone or handbells) are struck sharply to cause a vibration.
Period
The time it takes for a complete wave to pass. It is the inverse of the frequency (period = 1/frequency).
This is an example of a thread vibrating with one complete wave. Because it appears to stand still, it is called a standing wave.
1/2 wavelength
This is a thread vibrating with half of
a wave. Notice that the block has been moved so that the thead is shorter.
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