Waves

Waves

1. Properties of waves

   • parts of a wave
     • rest position - the undisturbed position of particles or fields when they are not vibrating
     • displacement - the distance that a certain point in the medium has moved from its rest position
     • peak - the highest point above the rest position
     • trough - the lowest point below the rest position
     • amplitude - the maximum displacement of a point of a wave from its rest position
     • wavelength - distance covered by a full cycle of the wave, usually measured from peak to peak, or trough to trough
     • time period - the time taken for a full cycle of the wave, usually measured from peak to peak, or trough to trough
     • frequency - the number of waves passing a point each second
   • All waves transfer energy but they do not transfer matter.

2. Transverse and longitudinal waves

   • Longitudinal waves
     • In longitudinal waves, the vibrations are parallel to the direction of wave travel.
     • Examples of longitudinal waves include: sound waves, ultrasound waves, seismic P-waves
     • One way to remember the movement of particles in longitudinal waves is to use the 'P' sound: longitudinal waves such as seismic P-waves may be thought of as pressure or push waves as the particles move parallel to the wave.
     • Longitudinal waves show areas of compression and rarefaction
       • Compressions are regions of high pressure due to particles being close together
       • Rarefactions are regions of low pressure due to particles being spread further apart
     • Longitudinal waves are often demonstrated by pushing and pulling a stretched slinky spring.
       • The compressions move from left to right and energy is transferred from left to right. However, none of the particles are transported along a longitudinal wave. Instead, they move backwards and forwards between compressions as the wave is transmitted through the medium.
   • Transverse waves
     • In transverse waves, the vibrations are at right angles to the direction of wave travel.
     • Examples of transverse waves include: ripples on the surface of water; vibrations in a guitar string; a Mexican wave in a sports stadium; electromagnetic waves - eg light waves, microwaves, radio waves; seismic S-waves
     • One way to remember the movement of particles in transverse waves is to use the 'S' sound: transverse waves such as seismic S-waves may be thought of as shake or shear waves as the particles move from side-to-side - crossing the direction of wave travel.
     • Transverse waves are often demonstrated by moving a rope rapidly up and down.
       • The rope moves up and down, producing peaks and troughs. Energy is transferred from left to right. However, none of the particles are transported along a transverse wave. The particles move up and down as the wave is transmitted through the medium.
     • Electromagnetic waves are transverse waves. Their vibrations or oscillations are changes in electrical and magnetic fields at right angles to the direction of wave travel.
       • All electromagnetic waves: transfer energy as radiation from the source of the waves to an absorber, can travel through a vacuum such as in space, travel at the same speed through a vacuum or the air
       • Electromagnetic waves travel at 300 million metres per second (m/s) through a vacuum.
       • The Electromagnetic spectrum: Electromagnetic waves form a continuous spectrum of waves.
         • This includes: waves with a very short wavelength, high frequency and high energy, waves with a very long wavelength, low frequency and low energy
         • Electromagnetic waves can be separated into seven distinct groups in the spectrum.
         • Each group contains a range of frequencies. For example, visible light contains all the frequencies that can be detected by the human eye: red light has the lowest frequencies of visible light violet light has the highest frequencies of visible light
           • The behaviour of an electromagnetic wave in a substance depends on its frequency. The differing behaviours of different groups in the electromagnetic spectrum make them suitable for a range of uses.
             • Radio waves are used for communication such as television and radio.
               • Radio waves are transmitted easily through air. They do not cause damage if absorbed by the human body, and they can be reflected to change their direction. These properties make them ideal for communications.
               • Radio waves can be produced by oscillations in electrical circuits. When radio waves are absorbed by a conductor, they create an alternating current. This electrical current has the same frequency as the radio waves. Information is coded into the wave before transmission, which can then be decoded when the wave is received. Television and radio systems use this principle to broadcast information.
             • Microwaves are used for cooking food and for satellite communications.
               • High frequency microwaves have frequencies which are easily absorbed by molecules in food. The internal energy of the molecules increases when they absorb microwaves, which causes heating. Microwaves pass easily through the atmosphere, so they can pass between stations on Earth and satellites in orbit.
             • Infrared light is used by electrical heaters, cookers for cooking food, and by infrared cameras which detect people in the dark.
               • Infrared light has frequencies which are absorbed by some chemical bonds. The internal energy of the bonds increases when they absorb infrared light, which causes heating. This makes infrared light useful for electrical heaters and for cooking food. All objects emit infrared light. The human eye cannot see this light but infrared cameras can detect it. This 'thermal imaging' is useful for detecting people in the dark.
             • Visible light is the light we can see. It is used in fibre optic communications, where coded pulses of light travel through glass fibres from a source to a receiver.
             • Ultraviolet: We cannot see ultraviolet light but it can have hazardous effects on the human body. Ultraviolet light in sunlight can cause the skin to tan or burn. Fluorescent substances are used in energy-efficient lamps - they absorb ultraviolet light produced inside the lamp, and re-emit the energy as visible light.
             • Electromagnetic waves in medicine: Changes in atoms and their nuclei can cause electromagnetic waves to be generated or absorbed. Gamma rays are produced by changes in the nucleus of an atom. They are a form of nuclear radiation. High energy waves such as X-rays and gamma rays are transmitted through body tissues with very little absorption. This makes them ideal for internal imaging. X-rays are absorbed by dense structures like bones, which is why X-ray photos are used to help identify broken bones.
               • Ionising radiation: Ultraviolet waves, X-rays and gamma rays are types of ionising radiation. They can add or remove electrons from molecules, producing electrically charged ions. Ionisation can have hazardous effects on the body: ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer x-rays and gamma rays can cause the mutation of genes, which can lead to cancer
                 • Radiation dose is a measure of the risk of harm caused by exposing the body to ionising radiation. Radiation dose is measured in Sieverts (Sv). As radiation dose figures are generally small, they are usually given in millisieverts (mSv): 1000 mSv = 1 Sv Background radiation is all around us, all the time. Sources include: radioactive rocks in the Earth's crust cosmic rays from space man-made sources such as nuclear weapons fallout and nuclear accidents The level of background radiation and dose are affected by factors such as the jobs that people do and the places where people live.

3. Reflection and refraction

   • Reflection of waves:
     • Waves - including sound and light - can be reflected at the boundary between two different materials. The reflection of sound causes echoes.
     • The law of reflection states that: angle of incidence = angle of reflection For example, if a light ray hits a surface at 32°, it will be reflected at 32°.
       • The angles of incidence and reflection are measured between the light ray and the normal - an imaginary line at 90° to the surface. The diagrams show a water wave being reflected at a barrier, and a light ray being reflected at a plane mirror.
     • Specular reflection: Reflection from a smooth, flat surface is called specular reflection.
       • This is the type of reflection that happens with a flat mirror. The image in a mirror is: upright, virtual
     • Diffuse reflection If a surface is rough, diffuse reflection happens. Instead of forming an image, the reflected light is scattered in all directions.
       • This may cause a distorted image of the object, as occurs with rippling water, or no image at all. Each individual reflection still obeys the law of reflection, but the different parts of the rough surface are at different angles.
   • Refraction of waves
     • Different materials have different densities. Light waves may change direction at the boundary between two transparent materials. Refraction is the change in direction of a wave at such a boundary.
       • The density of a material affects the speed that a wave will be transmitted through it. In general, the denser the transparent material, the more slowly light travels through it. Glass is denser than air, so a light ray passing from air into glass slows down. If the ray meets the boundary at an angle to the normal, it bends towards the normal. The reverse is also true. A light ray speeds up as it passes from glass into air, and bends away from the normal by the same angle.
     • Refraction can cause optical illusions as the light waves appear to come from a different position to their actual source.
     • A useful way of remembering the speed and direction changes of light during refraction is ‘FAST’: Faster - Away / Slower - Towards.
     • Wave speed, frequency and wavelength in refraction
       • For a given frequency of light, the wavelength is proportional to the wave speed: wave speed = frequency × wavelength
         • So if a wave slows down, its wavelength will decrease. The effect of this can be shown using wave front diagrams like the one below. The diagram shows that as a wave travels into a denser medium, such as water, it slows down and the wavelength decreases. Although the wave slows down, its frequency remains the same, due to the fact that its wavelength is shorter.
         • In this diagram, the right hand side of the incoming wave slows down before the left hand side does. This causes the wave to change direction.

4. Sound and ultrasound

   • Sound waves
     • Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases. The speed of sound depends on the medium through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuum because there are no particles to carry the vibrations.
     • The ear The human ear detects sound. Sound waves enter the ear canal and cause the eardrum to vibrate. Three small bones transmit these vibrations to the cochlea. This produces electrical signals which pass through the auditory nerve to the brain, where they are interpreted as sound.
       • The cochlea is only stimulated by a limited range of frequencies. This means that humans can only hear certain frequencies. The range of normal human hearing is 20 Hertz (Hz) to 20,000 Hz (20 kHz).
     • Properties of sound
       • The frequency of a sound wave is related to the pitch that is heard: high frequency sound waves are high pitched, low frequency sound waves are low pitched
       • The amplitude of a sound wave is related to the volume of the sound: high amplitude sound waves are loud low amplitude sound waves are quiet
   • Ultrasound
     • Ultrasound waves have a frequency higher than the upper limit for human hearing - above 20,000 Hertz (Hz). Different species of animal have different hearing ranges. This explains why a dog can hear the ultrasound produced by a dog whistle but humans cannot.
     • Uses of ultrasound include: breaking kidney stones, cleaning jewellery
       • In both of these applications, the vibrations caused by the ultrasound shake apart the dirt or kidney stones, breaking them up. The principle is the same as the opera singer's trick, where a glass may shatter if the singer makes a high-pitched sound near to the glass.
     • Ultrasound imaging creates a picture of something that cannot be seen directly, such as an unborn baby in the womb, or faults and defects inside manufactured parts.
       • These uses rely on what happens when ultrasound waves meet the boundary between two different materials. When this happens: some of the ultrasound waves are reflected at the boundary, the time taken for the waves to leave a source and return to a detector is measured, the depth of the boundary can be determined using the speed of sound in the material and the time taken
     • Echo sounding: High frequency sound waves can be used to detect objects in deep water and to measure water depth. The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object. The process is very similar to ultrasound imaging. However, the sound waves used are within normal hearing range, and they are used to identify objects rather than internal structures.
       • This technique is applied in sonar systems used to find shipwrecks, submarines and shoals of fish. Bats and dolphins use a similar method called 'echolocation' to detect their surroundings and to find food.

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