Waves

Waves

1. Types of Waves

   • Waves can be longitudinal or transverse
   • In transverse waves, the oscillations are at right angles to the direction of travel and energy transfer. Light and other types of electromagnetic radiation are transverse waves. All types of electromagnetic waves travel at the same speed through a vacuum, such as through space. Water waves and S waves are also transverse waves.
   • In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer. Sound waves and waves in a stretched spring are longitudinal waves. P waves are also longitudinal waves. Longitudinal waves show areas of compression and rarefaction. In the slideshow below, the areas of compression are where the parts of the spring are close together, while the areas of rarefaction are where they are far apart.
   • Seismic waves travel through the layers of the Earth. They usually result from an earthquake, volcanic eruption or other explosion.
     • Seismic waves can be reflected and refracted at boundaries between the crust, mantle and core. This means that they change direction.
     • Earthquakes are detected using a seismometer – a piece of equipment that picks up the vibrations in the ground.
     • P waves Longitudinal. Fast moving. Travel through liquids and solids only.
     • S waves Transverse. Slow moving. Travel through solids only.

2. Features of Waves

   • Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred.
   • Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium, and it is the medium that vibrates as the waves travel through.
   • Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves, and other types of electromagnetic radiation, are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.
   • As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. It is important to note that the amplitude is not the distance between the top and bottom of a wave.
   • The wavelength of a wave is the distance between a point on one wave and the same point on the next wave.
   • The frequency of a wave is the number of waves produced by a source each second.
   • The period of a wave is the time taken for one wave to be produced.
   • This is an imaginary surface that we draw to represent the vibrating part of a wave. If you draw semi-circular sound waves spreading out from a speaker, the semi-circular lines are the wavefront.
   • When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is diffraction.

3. Reflection and Refraction

   • Light and sound both travel as waves, but they are not identical.
   • Diffraction is the spreading out of waves when they pass through a gap. The extent to which this happens depends on the wavelength and size of the gap.
   • Sound waves and light waves reflect from surfaces. The angle of incidence equals the angle of reflection. This is called the law of reflection.
     • Both angles are measured from the normal. The normal is an imaginary line at right angles to the mirror.
     • Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images.
       • Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.
   • Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction. We can use water waves in a ripple tank to show this effect.
     • Refraction doesn't happen if the waves cross the boundary at an angle of 90° - in this case, they carry straight on.
     • At a specific angle, the light ray will no longer leave the block. At this point the angle of incidence is called the critical angle. Any further increase in the angle of incidence will mean the ray is reflected, not refracted.
     • When white light passes from air into a triangular prism, it is refracted as it enters, and then again as it exits. As it leaves the prism, the different wavelengths of the individual colours of light result in different angles of refraction.
       • This splits white light into the seven colours of the rainbow. This process is called dispersion. Red light is refracted the least and violet the most. Each colour of light can be called monochromatic.
   • An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.
     • Optical fibres are used in endoscopes that allow surgeons to see inside their patients. Optical fibres can also carry enormous amounts of information as pulses of light.

4. Sound

   • Sound waves are longitudinal waves which means they vibrate in the same direction as the direction of travel. They are produced by vibrating sources, such as speakers. Sound waves can only travel through a solid, liquid or gas medium. They travel fastest in solids, then liquids and slowest in gases. A good model for sound waves is a spring.
   • The sections of the sound waves where the particles are pushed together are areas of compression. The sections of the sound waves where the particles are further apart are areas of rarefaction. Sound waves can be reflected to form an echo, refracted and diffracted.
   • The speed of sound in air is about 340 m/s. This is much less than the speed of light in air which is about 300,000,000 m/s. This explains why we see lightning before hearing thunder. The speed of sound in water is about 1560 m/s. The speed of sound in solids varies depending upon the solid. It is about 1600 m/s in rubber and about 5000 m/s in steel.
   • Ultrasound is the name given to sound waves that have frequencies greater than 20,000Hz
     • When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time it takes for an ultrasound wave to leave the source, and bounce back to the detector. The further away the boundary, the longer the time between leaving the source and reaching the detector.
     • Computers are able to create detailed images by combining many ultrasound reflection readings. This is used in medicine for pre-natal scanning (checking unborn babies). When ultrasound is used for scanning babies, the distances and times are much smaller than those in the example above. Ultrasound can be used in industry for quality control procedures to check manufactured objects, such as railway tracks and oil pipelines, for damage or defects. The diagram shows how a piece of metal may be tested for cracks or other flaws using ultrasound:
   • An oscilloscope is a machine that shows the wave shape of an electrical signal. When connected to a microphone they can show the wave shapes of sounds.
   • Music and speech vary continuously in frequency and amplitude. In the same way, analogue signals can vary in frequency, amplitude, or both. FM (Frequency Modulated) and AM (Amplitude Modulated) are two types of radio signals. The diagram shows a typical oscilloscope trace of an analogue signal.
   • Digital signals are a series of pulses consisting of two states: ON (1) or OFF (0). There are no values in between. DAB (Digital Audio Broadcast) radio is transmitted as digital signals. The diagram shows a typical oscilloscope trace of a digital signal.
   • Data Transmission
     • All signals become weaker as they travel long distances. They may also pick up random extra signals. This is called noise. It is heard as crackles and hiss on radio programmes. Noise affects both analogue and digital signals.
     • Noise adds extra random information to analogue signals. Each time the signal is amplified, the noise is also amplified. Gradually, the signal becomes less and less like the original signal. Eventually, it may be impossible to make out the music in a radio broadcast against the background noise, for example.
     • Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the amplitude of the ON states. As a result, the electronics in the amplifiers can ignore the noise and it does not get passed along. This means that the quality of the signal is maintained, which is one reason why television broadcasters have changed from analogue to digital and radio broadcasters are in the process of changing. Because digital signals can carry more information per second than analogue signals, higher quality programmes or more channels can be broadcast. Another advantage of digital signals is that information can be stored and processed by computers.