If the number of cycles in a pulse increases while the wavelength remains constant, which statement is true?

• A. The pulse duration is increased
• B. The frequency is increased
• C. The propagation speed increases
• D. The period decreases

Answer: (A)

The pulse duration increases. The time span of a pulse is basically the number of cycles it contains multiplied by the period of one cycle. If the wavelength is fixed and the pulse travels in the same medium, the propagation speed stays the same, so the frequency (f = v/λ) remains unchanged and the period (T = 1/f) stays the same. Adding more cycles into the pulse simply makes the pulse last longer in time, increasing its duration. The propagation speed and the period do not change, so statements about speed increasing or period decreasing aren’t correct.