有關超聲波的應用的問題

This is because of "diffraction". When a wave encounters with an obstacle on its path of propagation, the wave will be "blocked" by the obstacle, thus forming a "shadow' behind the obstacle. However, if the dimension of the obstacle is comparable or even smaller than the wavelength of the wave, diffraction effect is prominent. That is, the wave will converge after passing through the obstacle and no "shadow" is formed.

For an obstacle of dimension a, say, diffractrion effect is prominent when the rastio 入/a (入 is the wavelength of the wave) approaches unity. Hence, to have a small obstacle (e.g. a tumour in the case of medical ultrasound) be detected, the wavelength of the wave used should be short.

For example, using ultrasound of frequency 2 MHz, the wavlength of the wave in soft tissue is around 0.8 mm (the speed of ultrasound in soft tisse is about 1540 m/s), which is sufficient to detect the presence of a tumour. If lower frequency ultrasound wave is used, e.g. 500 kHz, the smallest obstacle to be detected will then be around 3 mm, which is not good enough to detect an early tumour.