Concepts and the Frequency Domain
A signal can be viewed from two different standpoints:

The frequency domain

The time domain
In astronomy the frequency domain is perhaps the most familiar,
because a spectrometer, e.g. a prism or a diffraction grating, splits light
into its component color or frequencies and permits us to record
its spectral content. This is like the trace on a spectrum analyzer, where
the horizontal deflection is the frequency variable and the vertical deflection
is the signals amplitude at that frequency.
In the lab we are also familiar with the time domain. This is
like the trace on an oscilloscope where the vertical deflection is the
signals amplitude, and the horizontal deflection is the time variable.
Any signal can be fully described in either of these domains. We can
go between the two by using a tool called the Fourier transform.
Why the frequency domain ?
Depending on what we want to do with the signal, one domain tends to be
more useful than the other, so rather than getting tied up in mathematics
with a time domain signal we might convert it to the frequency domain where
the mathematics are simpler.
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