Waves

Waves

1. Transverse and Longtitudinal

   • Longtitudinal
     • 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.
     • Examples of longtitudinal waves
       • sound waves
       • ultrasound waves
       • P-type earthquakes
     • A longitudinal wave is one in which the vibrations of the particles are parallel to the direction in which the energy of the wave travels.
   • Transverse
     • A transverse wave is one in which the vibrations of the particles are at right angles to the direction in which the energy of the wave travels.
     • Examples of transverse waves
       • ripples on the surface of water
       • vibrations in a guitar string;
       • a Mexican wave in a sports stadium;
       • electromagnetic waves
         • light waves
         • microwaves
         • radio waves
         • x-rays
       • S type earthquake waves

2. Frequency, wavelength and amplitude

   • frequency (f) - the number of waves passing a point each second. Frequency is measured in hertz Hz. A frequency of 50 Hz means there are 50 vibrations every second.
     • most people cannot hear a high-pitched sound above 20 kHz
     • radio stations broadcast radio waves with frequencies of about 100 MHz
     • most wireless computer networks operate at 2.4 GHz.
     • Frequency = Number of waves to pass a point ÷ Time taken in seconds
   • wavelength (λ) - distance covered by a full cycle of the wave, usually measured from peak to peak, or trough to trough. Wavelength is measured in m.
   • amplitude - the maximum displacement of a point of a wave from its rest position.
   • Wave Speed
     • wave speed = frequency x wavelength
       • V = F x λ
       • All waves, including sound waves and electromagnetic waves, follow this equation.
       • some particular waves have their own specific speeds:
         • the speed of light and all of the electromagnetic spectrum in a vacuum is 300,000,000 m/s or 3×108 m/s
         • the speed of light and all of the electromagnetic spectrum in a vacuum is 300,000,000 m/s or 3×108 m/s

3. Echoes, sonar, and radar

   • Ultrasound
     • Ultrasound waves are sound waves which have a frequency higher than the upper limit for human hearing - above 20,000 Hz.
     • Ultrasound waves are longitudinal because they are simply high frequency sound waves i.e. above 20 kHz.
     • Uses
       • Ultrasound imaging creates a picture of something that cannot be seen directly, such as an unborn baby in the womb (a foetus), or faults and defects inside metals.
       • Ultrasound imaging helps to diagnose problems with the: heart, kidneys, blood vessels, bladder.
       • The best known non-imaging medical use of ultrasound is the breaking of kidney stones. The vibrations caused by the ultrasound shake apart the kidney stones, breaking them up.
     • Advantages
       • ultrasound waves pass through tissue without causing harm, unlike x-rays which cause ionisation and can damage DNA inside cells
       • ultrasound equipment is relatively cheap, portable and easy to use
       • images of internal organs can be seen without having to operate on patients.
   • 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.
   • Echoes
     • Sound waves can reflect off surfaces. We hear reflected sound waves as echoes.
     • Sound travels at a constant speed in a medium.
     • speed = distance ÷ time
     • Remember in the time taken to hear an echo the sound has travelled to the object and back again. To calculate the distance to the object use half of the time (or calculate half the total distance there and back)

4. Electromagnetic Waves

   • Electromagnetic waves are transverse waves with a wide range of different properties and uses.
   • Properties
     • are transverse waves
     • can travel through a vacuum
     • travel at exactly the same speed in a vacuum the speed of light, 300,000,000 m/s.
     • transfer energy from one place to another
     • can be reflected
     • can be refracted
   • Dangers
     • radio waves in large doses are believed to cause cancer, leukaemia and other disorders, and some people claim the very low frequency radio waves from overhead power cables near their homes has affected their health, although this has not been reliably proven;
     • microwaves can cause internal heating of body tissues;
     • infrared radiation is felt as heat and causes skin to burn
     • visible light from a laser which is very intense can damage the retina at the back of the eye;
     • ultraviolet can damage skin cells and lead to skin cancer and damage to eyes;
     • X-rays damage cells inside the body. They cause dangerous ionisation and when this happens with molecules in living cells, the genetic material of a cell, the DNA is damaged. This can lead to cancer. This is why doctors and dentists stand behind protective screens when taking lots of X-rays
     • gamma rays also damage cells inside the body causing dangerous ionisation in living cells which damages DNA. This can lead to cell death and cancer.

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