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Assume that a radio station is about $200 \mathrm{~km}$ away from your location and the station operates at $972 \mathrm{kHz}$. How long does it take for an electromagnetic signal to travel from the station to you and how many wave crests does it send out per second?
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The correct answer is:
$666 \mu \mathrm{s}$ and $9.72 \times 10^5$ crests per second
Time taken by wave to travel
$=\frac{200 \mathrm{~km}}{3 \times 10^5 \mathrm{~km} \mathrm{~s}^{-1}}=6.667 \times 10^{-4} \mathrm{~s} \approx 666 \mu \mathrm{s}$
Number of crests per second $=$ Frequency
Frequency $=972 \mathrm{kHz}=9.72 \times 10^5 \mathrm{~Hz}$
$\therefore \quad$ There are $9.72 \times 10^5$ crests per second.
$=\frac{200 \mathrm{~km}}{3 \times 10^5 \mathrm{~km} \mathrm{~s}^{-1}}=6.667 \times 10^{-4} \mathrm{~s} \approx 666 \mu \mathrm{s}$
Number of crests per second $=$ Frequency
Frequency $=972 \mathrm{kHz}=9.72 \times 10^5 \mathrm{~Hz}$
$\therefore \quad$ There are $9.72 \times 10^5$ crests per second.
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