Sound and light

We see simple light waves (those with only one frequency) as one of the colours of the rainbow. Combining together two or more sine waves of light produces a more complex colour. The more frequencies you add, the whiter the resulting colour gets. With light, you can easily separate the frequencies by shining the complex light wave through a prism.

The set of frequencies in a light wave (as separated by a prism) is called its spectrum.

Scientists can identify different substances by looking at the spectrum of the light that the substances emit when they're heated. Iron will glow with a different set of frequencies than nickel or sulphur.

The situation is similar with sound. The complex sound wave for an [i] vowel will be composed of a different pattern of frequencies than the complex wave for an [a].

We need a way to separate a complex sound wave out into its component frequencies (and their amplitudes) so that we can see what makes vowels different from each other. A spectrograph machine is essentially just a prism for sound.

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