Frequency from CRO waveform period

Steps:

• Time base of 1 cm ms⁻¹ means 1 ms per cm on the horizontal axis.
• The CRO trace shows one complete sound wave cycle spanning 4 cm horizontally.
• Period T = 4 cm × 1 ms/cm = 4 ms = 0.004 s.
• Frequency f = 1/T = 1/0.004 s = 250 Hz.

Why A is correct:

• Frequency equals the reciprocal of the period, where period is the time for one cycle as scaled by the CRO time base.

Why the others are wrong:

• B: 500 Hz requires T = 2 ms (2 cm/cycle), but trace shows 4 cm/cycle.
• C: 670 Hz requires T ≈ 1.5 ms (1.5 cm/cycle), but trace shows 4 cm/cycle.
• D: 4000 Hz requires T = 0.25 ms (0.25 cm/cycle), but trace shows 4 cm/cycle.

Final answer: A