What period corresponds to $$ 500 Hz$$?
Definitions:
Hz=cycles/second
Period = time required for 1 cycle
$$ 500 Hz=\frac{500 cycles}{second}$$
or written in an equivalent form:
$$ 500 Hz \rightarrow $$ a ratio of $$ 500 $$ cyles:$$ 1 $$ second
or $$ 1$$ cycle: $$ \frac{1}{500}$$ second
So the period is $$ \frac{1}{500}=0.002$$ second.