Period of a 40 Hz waveform?

• A. 0.4 s
• B. 0.0025 s
• C. 2.5 s
• D. 0.025 s

Answer: (D)

Period is the time for one complete cycle, which is the reciprocal of frequency: T = 1/f. For a 40 Hz waveform, one cycle takes 1/40 second, which equals 0.025 s (25 milliseconds). This matches having 40 cycles in one second. If the period were 0.4 s, that would be 1/0.4 = 2.5 Hz; if it were 0.0025 s, that would be 400 Hz; if it were 2.5 s, that would be 0.4 Hz.