Reflection, Sound Waves and Waves for Detection
Reflection of Waves
When a wave hits a boundary between two different materials, it can be reflected, absorbed or transmitted. The law of reflection states that the angle of incidence equals the angle of reflection. Both angles are measured from the normal — an imaginary line drawn at right angles to the surface at the point of incidence.
Specular reflection occurs on smooth surfaces (like mirrors), where all the reflected rays are parallel, producing a clear image. Diffuse reflection occurs on rough surfaces, where the reflected rays scatter in many directions because the normal is different at every point on the surface. Most objects are seen by diffuse reflection of light.
The law of reflection applies to all waves (light, sound, water). The angle of incidence always equals the angle of reflection, measured from the normal to the surface.
Sound Waves
Sound waves are longitudinal waves produced by vibrating objects. They need a medium (solid, liquid or gas) to travel through — they cannot travel through a vacuum. In a sound wave, particles vibrate back and forth along the direction of energy transfer, creating regions of high pressure (compressions) and low pressure (rarefactions).
The speed of sound depends on the medium. Sound travels fastest in solids (about 5000 m/s in steel), slower in liquids (about 1500 m/s in water) and slowest in gases (about 340 m/s in air at room temperature). This is because particles are closer together in solids, so vibrations pass from one particle to the next more quickly.
The human ear can detect sounds with frequencies between approximately 20 Hz and 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasound, and sounds above 20 kHz are called ultrasound. The human hearing range decreases with age — older people typically cannot hear the highest frequencies.
The pitch of a sound depends on its frequency (higher frequency = higher pitch), while the loudness depends on its amplitude (greater amplitude = louder sound).
Ultrasound for Detection and Imaging
Ultrasound has many practical applications. In medical imaging, ultrasound waves are sent into the body. When they reach a boundary between two different tissues (for example, between fluid and solid tissue), some of the wave is reflected back. A computer analyses the reflected pulses to build an image of internal structures. This is commonly used for prenatal scans to monitor the development of a baby during pregnancy.
Ultrasound is preferred over X-rays for prenatal scans because it does not use ionising radiation, making it safer for the developing baby.
Ultrasound is also used in industry to detect cracks and flaws inside metal structures without cutting them open (non-destructive testing). A pulse of ultrasound is sent into the metal, and any internal crack or defect will reflect some of the wave back earlier than the far boundary, showing up on a display.
Echo sounding (sonar) uses ultrasound to measure the depth of water or locate objects underwater. A pulse of ultrasound is sent from a ship towards the seabed. The time taken for the echo to return is measured. The depth is calculated using:
$$\text{distance} = \frac{\text{speed} \times \text{time}}{2}$$
The division by 2 accounts for the fact that the pulse travels to the seabed and back. The same principle is used by bats and dolphins for echolocation — emitting ultrasound pulses and listening for reflections to navigate and find prey.
Seismic Waves for Detection
Seismic waves are produced by earthquakes and travel through the Earth. P-waves (primary waves) are longitudinal and travel through solids and liquids. S-waves (secondary waves) are transverse and can only travel through solids — they cannot pass through liquids. By analysing how seismic waves travel through the Earth, scientists have deduced the internal structure of the planet.
The fact that S-waves are not detected on the opposite side of the Earth from an earthquake (in the “shadow zone”) tells us that the Earth has a liquid outer core. P-waves are refracted as they pass through different layers, providing evidence for the density changes at the mantle-core boundary.
P-waves are longitudinal (particles vibrate parallel to the direction of travel), travel through solids and liquids, and are faster than S-waves — they arrive at seismometers first (hence “primary”). S-waves are transverse (particles vibrate perpendicular to the direction of travel), travel through solids only, and are slower (hence “secondary”). Both types are refracted when they pass between layers of different density.