Waves can be reflected, refracted, transmitted or absorbed at a boundary. Refraction happens because a wave changes speed when it enters a different medium; if it meets the boundary at an angle, it changes direction. Light slows down entering glass and bends towards the normal; leaving glass it speeds up and bends away. Each colour refracts by a slightly different amount, which is why a prism spreads white light into a spectrum.
Sound waves are longitudinal; the human ear hears about 20 Hz to 20 kHz. Ultrasound (above 20 kHz) is partly reflected at boundaries between tissues, so the time of echoes builds an image for medical scans and finds flaws in metals. Seismic P-waves are longitudinal and travel through solids and liquids; S-waves are transverse and cannot pass through liquids. The S-wave shadow zone is evidence for the Earth's liquid outer core.
Worked example
An ultrasound pulse returns from a boundary after 0.0002 s. Its speed in tissue is 1500 m/s. How deep is the boundary?
- Distance = speed × time = 1500 × 0.0002 = 0.3 m.
- That is there and back.
Answer:Depth = 0.15 m (15 cm).
Key words
- refraction
- change in direction due to change in speed
- normal
- line at 90° to a boundary
- ultrasound
- sound above 20 kHz
- P-waves and S-waves
- longitudinal and transverse seismic waves
New questions every time.
NGSS-referenced: HS-PS4-1