Standing Waves for Wavelength Measurement

Steps:

• Sound reflects back from CRO plate, creating interference and standing waves along the path.
• Quiet spots are nodes where destructive interference occurs; distance between consecutive nodes is λ/2.
• Measure mean separation d of quiet spots; wavelength λ = 2d.
• No other measurements required for λ using this method.

Why D is correct:

• In standing sound waves, node separation equals λ/2, so λ = 2 × mean separation of quiet spots (standing wave node definition).

Why the others are wrong:

• A: Frequency is unnecessary; separation alone gives λ via standing wave relation.
• B: Speed and frequency allow λ = v/f, but ignores stationary wave method.
• C: Identical to B; uses wave speed formula, not node measurements.

Final answer: D