Waves

• • Transverse waves

  o The oscillations are perpendicular to the direction of energy transfer e.g.

  ▪ Electromagnetic waves e.g. light

  ▪ Ripples on a water surface

  ▪ A wave on a string
• • Longitudinal waves

  o The oscillations are parallel to the direction of energy transfer e.g.

  ▪ Sound waves in the air

  ▪ Shock waves e.g. some seismic waves
• • When waves travel through a medium, particles of the medium oscillate & transfer energy between each other

  • All waves transfer energy but not matter

  o This can be proved by putting an orange in the water than creating waves, the orange will move up & down be remain in the same spot
• • Mechanical waves

  o cause oscillations of particles in a solid, liquid or gas & must have a medium to travel through

  • Electromagnetic waves

  o cause oscillations in electrical and magnetic fields
• • Amplitude – maximum displacement from its undisturbed position

  • Wave speed –speed at which the energy is transferred (or the wave moves) through the medium

  • Wavelength – distance between the same point on 2 adjacent waves e.g. trough to trough (measured in metres)

  • Frequency – number of complete waves passing a point per second (1Hz is 1 wave per second)
• • Period – amount of time it takes for a full cycle of the wave
• • Relationships

  o Increase frequency, velocity increases

  o Wavelength increases, velocity increases

  o Period is inversely proportional to frequency

  o Smaller period, higher frequency, greater velocity
• at a boundary between 2 different materials the waves can be: reflected, absorbed, or transmitted
• Transmission

  • Waves will pass through a transparent material

  • The more transparent, the more light will pass through the material

  • It can still refract, but the process of passing through the material & still emerging is transmission
• Absorption

  • If the frequency of light matches the energy levels of the electrons

  o the light will be absorbed by the electrons & not reemitted

  o energy is transferred to the material’s energy stores

  o they will be absorbed, and then reemitted over time as heat

  o so that particular frequency has been absorbed

  • If a material appears green, only green light has been reflected - the rest of the frequencies in visible light have been absorbed
• Reflection

  • The rule of reflection – angle of incidence = angle of reflection

  • The angles of incidence & reflection are the angles where the ray meets the normal

  • The normal is the imaginary line perpendicular to the point of incidence
• • Light will reflect if the object is opaque & is not absorbed by the

  material

  o The electrons absorb the light energy then reemit it as a reflected wave

  • The smoother the surface, the stronger the reflected ray

  o A mirror gives clear reflections, the reflected ray is thin & bright (same as the incident ray)

  • Rough surfaces scatter the light in all directions, so they appear matt & not reflective

  o The reflected ray will be wider & dimmer (or not observable at all)
• • Sound waves can travel through solids causing particles to vibrate in the solid

  o These vibrations are passed through the surrounding medium as a longitudinal wave

  o Sound travels faster in solids than liquids & faster in liquids than in gases

  o Sound can’t travel in space – as it is mostly a vacuum (no particles to vibrate)
• • Hearing

  o The outer ear collects the sound waves & channels it down the ear canal

  o The tightly stretched membrane vibrates as incoming pressure waves reach it

  ▪ Compression forces the eardrum outwards

  ▪ Rarefaction forces the eardrum inwards (due to pressure)
• o The eardrum & ossicle bones (connected to eardrum) vibrates at the same frequency as the sound wave

  ▪ Vibrations of the ossicle bones transmitted to the fluid in the inner ear - compression waves are

  thus transferred to the fluid (in the cochlea)
• o As the fluid moves due to the compression waves, the small hairs that line the cochlea move too

  ▪ Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies

  ▪ The hairs each come from a nerve cell

  o when a certain frequency is received, the hair attuned to that specific frequency moves a lot, releasing an electrical impulse to the brain

  ▪ causing the sensation of sound
• • Limitations

  o Conversion of sound waves to vibrations of solids has a limited range (different for every material)

  ▪ Restricting the limits of human hearing (20Hz to 20kHz)

  ▪ Microphones can pick up much wider frequencies – but we can’t here all of them

  ➢ Sound waves cause a diaphragm to vibrate & this movement is transfused into electrical

  signals
• o Human hearing is limited by the size & shape of out ear drum

  o In the cochlea, the hairs attuned to high frequencies can die/get damaged

  ▪ Due to constant loud noise, drugs, chemotherapy, changes in the inner ear due to growth

  ▪ As you get older, you can’t hear higher frequencies
• • Shorter wavelength = higher frequency = higher pitch

  • Higher amplitude = higher volume

  • Sound waves can be reflected by hard flat surfaces (echoes)

  • Sound waves are refracted as they enter different media

  o Entering a denser medium causes the wavelength to shorten but frequency increase so speed increases
• Ultrasound waves

  • Have a higher frequency than the upper limit of human hearing

  o Produced by electrical devices that produce electrical oscillations – easily converted into mechanical

  vibrations to produce sound over 20kHz

  • When a wave reached a boundary between 2 media, some of the wave is reflected & some is transmitted

  (refracted) – called partial reflection

  o if you point a pulse of ultrasound at an object, at the boundary some waves are reflected

  o the time taken for the reflections to reach a detector can be used to measure the distance
• • Ultrasound has 2 main uses:

  o Medical imaging e.g. pre-natal scanning of a foetus

  ▪ Ultrasound waves can pass through the body, but when they reach a boundary (e.g. fluid in the

  womb & the skin of the foetus there is partial reflection which can be detected

  ▪ Exact timing & distribution of the echoes are processed to produce a video image of the foetus

  ▪ The risks are not entirely known, but they are definitely safer than X-rays
• • Ultrasound has 2 main uses:

  o Industrial imaging e.g. finding flaws in materials (i.e. wood & metal) or pipes

  ▪ Ultrasound waves entering a material will usually be reflected by the far side of the material

  ▪ If there is a crack in the object, the wave will be reflected sooner
• Echo sounding (sonar)

  • Uses high frequency sound waves (including ultrasound)

  • Used by boats & submarines to find out the depth of the water or locate an object on the sea floor

  o a pulse of the soundwave is sent below a ship – the time taken for reflection can be used to calculate depth
• Seismic waves (infrasound - frequency lower than 20Hz)

  • produced by earthquakes

  o P-waves are longitudinal, different speeds through solids & liquids

  o S-waves are transverse, cannot travel through a liquid

  o S-waves are slower than P-waves

  o When they reach a boundary between 2 layers, some waves will be

  absorbed & some will be refracted (change speed gradually resulting in a curved path)

  o These seismic waves are detected using seismometers
• o Observing how seismic waves are absorbed & reflected allows scientists to understand the structure & size of the Earth’s core (new evidence)

  ▪ On the opposite side of the Earth to the earthquake, only P-waves are detected, suggesting the core of the Earth is liquid (as no S-waves are detected)
• S Waves and P Waves inside Earth

  
• • Electromagnetic waves are transvers waves that transfer energy from the source to an absorber

  • They form a continuous spectrum & all types of electromagnetic wave travel at the same velocity through a

  vacuum (space) or air

  o They don’t required particles to move – instead, they are vibrations of electric & magnetic fields

  o In space all electromagnetic waves travel at the speed of light

  o Grouped in terms of wavelength & frequency (7 groups)

  o Our eyes only detect visible light 0 so detect a limited range of EM waves

  
• As speed is constant for all EM waves

  o Wave length decreases, frequency must increase, so the energy of the wave increases
• • Different substances may absorb, transmit, refract, or reflect EM waves in ways that vary with wavelength

  o E.g. glass – interacts differently with different parts of the EM spectrum due to different wavelengths

  ▪ transmit/refract visible light

  ▪ absorb ultraviolet radiation

  ▪ reflect infrared radiation
• • Refraction is due to the difference in velocity of waves in different

  substances

  o If entering a denser material, the wave will bend towards the normal

  ▪ This is because the wave speed decreases

  ▪ higher density = lower wave speed

  o If entering a less dense material, the wave will bend away from the

  normal

  o Wavelength changes when a wave is refracted but the frequency

  stays the same

  • Shorter wavelengths slow down more than longer wavelengths

  o E.g. blue light slows down more than red light at

  the same boundary
• • If a wave is travelling along the normal, it will change speed but is not refracted

  • Optical density of a material is a measure of how quickly light can travel through it – higher value = slower travel

  • Prisms disperse white light to create a rainbow – each wavelength refracts a different amount, so they spread out to create a rainbow effect
• • EM waves are made up of oscillating electric & magnetic fields

  • Alternating currents are made up of oscillating charges, which produce EM waves

  o The frequency of the waves produced will be equal to the frequency of the AC

  o Radio waves are produced by oscillations in electrical circuits
• ▪ The object in which charges/electrons oscillate (to produce radio waves) is called the transmitter

  o When they are absorbed, the energy carried by the waves is transferred to the electrons in the receiver

  ▪ The energy causes the electrons to oscillate & if the receiver is part of a complete circuit, AC is generated with the same frequency as the radio wave itself
• • when electrons change orbit (move closer or further from the nucleus)

  o move to a higher orbit (further from nucleus)

  ▪ atom has absorbed EM radiation

  o fall to a lower orbit (closer to the nucleus)

  ▪ atom has emitted EM radiation

  • if an electron gains enough energy, it can leave the atom to form an ion

  • so, gamma rays originate from changes in the nucleus of an atom
• o UV light, X-rays and gamma can have hazardous effects on human body tissue

  o The effects depend on the type of radiation and the size of the dose

  ▪ Radiation dose: measure of the risk of harm resulting from an exposure of the body to radiation

  o Low frequency waves e.g. radio waves

  ▪ Don’t transfer much energy so pass through soft tissue without absorption

  o High frequency waves e.g. UV, X-rays & gamma rays

  ▪ Transfer large amounts of energy, can cause lots of damage
• o UV – damages surface cells

  ▪ Can cause skin to age prematurely & increases risk of skin cancer

  ▪ Sun cream prevents over-exposure in summer

  o X-ray & gamma rays

  ▪ these are ionisation radiation (carry enough energy to remove electrons from atoms) that can

  cause the mutation of genes or cell destruction – causing cancer

  ▪ Minimal exposure should be ensured
• Radio waves – communication (TV & radio)

  • Long-wave radio (1-10km wavelengths)

  o can be transmitted around half the world because the waves diffract (bend) around the curved surface

  of the Earth (also hills & tunnels etc.)

  o So, signals can be received without direct line of sight between transmitter & receiver
• • Short-wave radio (10-100m wavelengths)

  o can be received at long distances because they are reflected off the ionosphere (electrically charged

  layer in the Earth’s upper atmosphere)

  o E.g. Bluetooth – sends data over short devices wireless

  • Radio waves used for FM radio & TV have very short wavelengths – require direct sight of transmitter

  • Long wavelength – allows for long distance travelling without losing quality (energy)
• Microwaves – satellite communication & cooking food

  • Satellites - the microwaves used can penetrate Earth’s watery atmosphere

  o Allows them to reach satellites - so signals can be transmitted between Earth & satellite dishes

  thousands of kilometres above Earth (with a slight delay due to the distance)
• • Microwave ovens – the microwaves used need to be absorbed by water molecules in food

  o The microwaves can penetrate a few centimetres into the food – then they are absorbed & transfer

  energy to the water molecules in the food, causing the water to heat up

  o The water molecules then transfer energy to the rest of the molecules in the food by heating

  • They both use microwaves with different wavelengths