Question

(a) Describe the distinction between transverse and longitudinal waves. Give an example of each type ofwave, describing the disturbance in each case.

(b) Describe why the frequency of a wave perceived by an observer depends on her velocity.

(c) Ultrasound measurements in medicine are often conducted using wavelengths of 0.03 cm and can be used measure blood flow speeds via the Doppler effect. Given that the speed ofsound in the body is 1550 m/s, what is the maximum frequency difference between the transmitted and received signals for a blood flow velocity of 13 cm/s.

Answer 1

(a) Longitudinal waves :

1. A longitudinal waves is a wave where the movement of the medium is in the same direction as the wave.
2. This causes particles to move left and right inducing other particles to vibrate.
3. The longitudinal waves are one dimension.

Transverse waves :

1. Transverse waves is a wave where the movement of the medium is at a right angle to the wave direction.
2. The particles move up and down as the wave mones horizontally.
3. The transverse waves are the two dimension.

Example : Sound wave, the primary wave in the earthquake are examples of longitudinal waves. In these waves, the particles of the medium ( like air or any other medium ) through which the wave is travelling, make alternative compressions and rarefactions to transfer the wave energy from one place to another.

Light wave, ocean wave, the secondary wave in the earthquake are the examples of transverse waves. In this waves, the wave itself ( in case of light wave, photons ) or the particles of the medium ( like air or any other medium ) through which the wave is travelling, make alternative crests and troughs perpendicular to the direction of travelling of the wave to transfer the wave energy from one place to another.

(b) A wave perceived by an observer depends on her velocity because of Doppler shift in frequency or wavelength of the wave. Whenever there is a relative motion between the observer and the source of sound, two scanario may occur :

(i) We know, when speed of a wave is constant, frequency and wavelength of a wave are inversely proportional. Now, if the observer ( the sound source ) is approaching the sound source ( the observer ), the effective wavelength of the sound get reduced depending on the relative velocity between them. Hence, effective frequency is increased, keeping the speed of sound in the medium unchanged.

(ii) We know, when speed of a wave is constant, frequency and wavelength of a wave are inversely proportional. Now, if the observer ( the sound source ) goes away from the sound source ( the observer ), the effective wavelength of the sound is increased depending on the relative velocity between them. Hence, effective frequency is reduced, keeping the speed of sound in the medium unchanged.