Practice question

**Relative motion** changes effective wavelength encountered. For moving source approaching stationary observer, wavelength ahead λ' = (v - v_s)/f, so f' = v/λ' = f·v/(v - v_s) > f, basis for calculating apparent pitch shift in sound. Standing waves in a guitar arise from the interference of waves reflected at fixed ends (e.g., bridge and nut), producing discrete frequencies (harmonics) determined by string length. Using v = fλ and standing-wave condition fₙ = n v/(2L) or v/(4L) as applicable, calculation yields Due to reflection at boundaries, illustrating frequency-length-speed interdependence and quantization by boundaries.