# What function does the sign wave represent in sound and light?

Cyberice
I know that in electronics the sine-wave applies to AC voltage because it is constantly going from negative to positive, and back to negative again causing an analog wave of the rising and collapsing voltage and sometimes magnetic fields. But sound and light are supposed to create waves by just vibrating. You can't have a negative/positive vibration, and the medium in which it is traveling is not ionizing back and forth switching polarities to for a variation to be shown (which would need to be true for a sine wave to be formed). So why would they use a analog negative and positive wave to represent its function?

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Staff Emeritus
Gold Member
The sine function or sums of sins can be used to model all periododic functions. (Even square waves)

Consider the full sine expresion

F(t) = D0 + Asin( &tau; + t)

Here D0 is the offset
A is the amplitude
&tau; it the phase shift
t is the variable.

If D0 > A then the function will never be negitive. In electronis this could represent a AC ripple voltage riding on a DC potential.

Since light "waves" are periodic in nature they can be represented by sine waves. Look up information on Fourier Series to learn how ANY periodic function can be repersented as a sum of sine functions.

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mmwave
Integral's comments are good but I thought it might also help you to know that for light or radio waves the electric field grows to a maximum in one direction, decreases and then grows to a maximum in the opposite direction so in that sense there is a reversal of sign.

For example, in a vertically polarized light wave, the electric field is either pointing up or down as the wave comes toward you. The amplitude at one point in space over time is described by a sine function and the sign refers to whether it is pointing upwards or downwards.