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Physics (P.5. Waves)

Isobel Temple English

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Basics
- A wave is an ossilation that transferes energy from one point to another
  - Mechanical waves: are vibrations which travel through a medium
    - Meduim: refers to a material of a stubstance (it travels through matter)
      - Electromagnetic waves: waves capable of transmitting energy through a vaccume (no medium/matter)
        A wave can be mechanical/ electromagnetic and transverse/ longitudinal
        Transverse waves: the vibrations are perpendicular (90 degrees) to the way the waves travel (e.g. all electromagnetic waves, ripples in water)
        Longitudinal waves: the vibrations are parallel to the direction the wave travels (squashed up then stretched out)
        You can model it with water drop something in it transvers waves happen, the wave moves but the water does not
        Wave properties
        Amplitude: distance from the middle to the top (crest) or bottom (trough) of a wave
        Symbol: A
        Unit: Meters or Volts
        Wave length: distance from one point on a wave to the same point on the next wave
        Symbol: Upside down 'y' (lambda)
        Unit: Meters(m)
        Frequency: Number of waves/ ossilations per second
        Symbol: F
        Unit: Herts (Hz)
        Time period: The time from one wave to pass a given point
        Symbol: T
        Unit: Seconds, s
        Wave speed (m/s) = frequency (Hz) x wavelength (m)
Wave velocity
- Wave speed
  - Wave velocity (m/s) = frequency (Hz) x wave length (m)
  - 1kHz = 1000Hz
    - 1MHz = 1,000,000Hz
- Velocity of sound
  - To work it out record how long it takes you to hear an echo
    - Or use microphones and a computer (diagram not acccurate, sound is longitudinal)
      - Velocity of sound varies with pressure and temperature
Wave properties and uses
- When a wave hits a boundary between two media (material interface) 3 things can happen (it depends on the densities), it can be: reflected (an echo), transmitted (and possible refracted), absorbed
  - When the wave is absorbed it transferes energy to that materials energy store
    - If the densities are different then more of the sound will be absorbed
      - When a wave travels from one mediun to another it may carry on moving (be transmitted) but it's velocity may change and it's direction may change - moves away from norm line, (a line at 90 to the surface) (refraction)
        It may be reflected, this is where it isn't absorbed or transmitted so it is 'sent back' from the other material
        Sound is genrally faster in solids than liquids and faster in liquids than gasses
        Reflection
        Angle of incidence = angle of reflection
        The "angle" if measured from the norm. The norm is an imagenry line drawn by a dotted line that is at 90 degrees to the surface
        Reflection of visible light is what lets us see things, light rays reflect of smooth surface (e.g.mirror) in the same direction giving a clear image This reflection is called specular (all reflected in the same direction)
        Light reflected of rough surface (e.g.paper) goes in all directions, each ray hits the surface at a differnt angle (angle of incidenc still = angle of reflection). This reflection is called scattered. It happens becuase the norm is different for each ray of light. The surface will therefore look matt
        White light is made up of lots of different colurs of light, this don't split up when reflected
        You need to be able to draw ray diagrams. Draw the normaland then the angle of incidence and reflection- make sure they are the same size. Always add arrows to the rays
        Reflection investigation...
        Draw the surface line on apiece of paper, draw a dotted line at 90 degrees to it (normal line). Place a mirror on the surface line. Get a ray box and shine at norror, so it intersects at the surface line and the norm line. Trace the incidence line and the relected line. measure the angle. Repeat for diferent angles and colours of light. The angle should always be the same no matter what
        Refraction
        Refraction is when waves bend
        Waves travel at diferent speeds in materials with different densities, so when it crosses a boundary it changes speed
        The frequency of the wave stays the same, it means the wavelength changes. (wavelenght decreases if wave slows down, wavelenght increases if it speeds up
        If it hits it at an angle to the norm the change in speed (and wavelength) makes it bend- refraction. Greater change in speed= greater bend
        If the wave slows down= bends to normal
        Colours have slightly different wavelengths so when they enter a denser substance the shorter wave lenght slows down more and refracts more.
        Light ray travelling through more dense material= slows down= towards the normal
        Less dense material= speeds up= away from material
        When white light travels through a triangular prism you get a rainbow
        You can investigate this using a blue and red light box and a prism
Sound in solids and the ear
- Sound waves are caused by vibrating objects, which are passes around the surrondings as longitudinal waves
  - As it travels through a solid it causes vibration of particle
    - Sound waves can reflect or refract
- The outer ear (pinna and auditory canal) gather the sound wave and direct it to the ear drum
  - The ear drum then vibrates, these vibrations are passed onto tiny bones called your ossicles (through the semiciruclar cannal) to the cochlea
    - The cochlea turns the vibrations into electrical signal which are sent to the brain, the brian interprets them as sound
      - As you get older the upper limit of what you can hear decrease (you can hear less high pitched sounds), due to the wear and tare of the cochlea and the hairs inside it. The hairs inside the cochlea all have different lengths and resonate (vibrate) at diferent frequencies. As you get older the shorter hairs (which pick up higher sounds) are lost.
- Bell in a jar
  - If a bell was put in a jar and the all the air was pumped out (a vacuum), then the bell were then to ring you would not hear the sound as sound waves need a medium to travel through (e.g. air)
- Sound travels at different speeds through differen tmediums e.g. sound travels slower through air than it does through steel
  - This happens due to different densities (how much matter) . Sound transferes energy by vibrations of the medium. The closer togetehr the particles, the more efficient (quicker) the transfer of energy (the faster the sound)
- Sound can sound different due to these things
  - Pitch: how high or low a tone is (change frequency)
    - Voulme: how loud or quite it is (change amplitude)
      - Amplitude: how high the wave is
    - Frequency: how often
Electromagnetic spectrum
- There are different waves (bands of frequencies) in the electromagnetic spectrum
  - The electromagnetic spectrum is a continuous spectrum
    - Our eyes are only sensitive to a narrow range of this, called visible light, different colours have different wave lengths
      - EM waves are transverse waves and they travel at the same speed through air or space (a vacuum), they travel at different speeds through different materials
        The waves with a shorter wave length have a higher frequency. And the higher the frequency the more energy transfered
        All EM waves transfer energy from a source to an absorber
Use and dangers of EM radiation
- Raidio waves
  - They are refracted by some layers of the atmosphere
    - They are transmitted through the body without being absorbed
      - They are used for communication, they are used to transmit imformation like television and radio shows
        TV and FM radio use very short wave length radio waves, other radio transmissions use medium and long wave.
        Penetration: They can go through soilds and gasses
        Hazards: There are no hazards that we know of at the moment
- Micro waves
  - Aren't refracted by layers of the atmosphere, making it good for satellite communication
    - Some wavelengths can be absorbed, it can cause heating of cells, which can be dangerous
      - Bluetooth uses microwaves as well as satellites (can pass easily through the earths very watery atmosphere) and wi-fi
        They are used for cooking and communications
        Used to cook food- the waves can penetrate a few cm then they are absorbed and the energy is transfered to water molecules, causing the water to heat up. Water molecules transfer thsi energy to the rest of the food- cooks food quickly
        Penetration: can't go through solids
        Hazards: can damage tissue
- Infra-red (IR)
  - Reflected and absorbed by the skin, may cause burns if the skin gets too hot
    - Pulses of infra-red can be used to communicate with TV (e.g. in remotes)
      - It can also be used to send imformation at high speeds using optical fibres
        IR radiation is given of by all objects, the hotter the object = the more IR radiation. Infra-red cameras can detect it and turn it into an electrical signal which is displayed on screen
        It can be sued for night- vision cameras (CCTV)
        Can be used in medicine to detect an increase in temperature around a wound. Use a thermal imaging camera to produce a thermogram
        Absorbing IR radiaion cause it to get hotter- so it can be sued to cook food
        Penetration: can't go through solids
        Hazards: too much can burn skin
- Visible light
  - Reflected and absorbed by the skin
    - Ships can use it to communicate e.g. morse code
      - It can be sued for cameras and is used in eyesight
        Light is used in optical fibres, which are used for telephones, internet cables and medical procedures
        Hazards: none
        Lasers, CD's, DVD's and Blu-ray discs use visible light
- Ultra-violet (UV)
  - It is absorbed by the skin. But it has a higher frequency so transfers more energy and therfore cause more damage. When it enters cells it collides with atoms in molecules, it may knock and electron off and cause ionisation (it is ionising radiation)- damages cells = genetic mutation, cancer, tissue damage, radiation sickness
    - If your eye is exposed to UV light it can cause catztacts, where your cornea goes blury
      - It helps to produce vitamin D which you need for strong bones
        Also forensic scientists use the facts that bodily fluids glow in UV light
        Can be used to detect fake bank notes
        Kills bacteria in water (sterilises it)
        It is fluresant (radiation is absorbed and then visible light is emmited)- it is an energy efficient way of producing light for long periods fo time
        Security pens can be used to mark propery so if it is stolen it can be easily identified
        Penetration: it can't go through soilds (e.g.suncream), it can go through air but not the Ozone layer
- X-rays
  - It is ionising (lead to cancer) x-rays have an even higher frequency than UV so can cause more damage and may get into deeper tissue
    - X-rays can kill cancer or other cells
      - Bones absorb many x-rays, but soft tissue (e.g. skin, mussle) does not. Photographic film darkens when x-ray hits it. A person can ly on photographyic film and have x-ray sent at them this will produce and image of the bone, to see if it is broken
        To protect people from radiaton they can stand behind leed walls
        A computer can also be used to do this, it produces an image that looks liek a slic ethrough your body- computerised tomography (CT scan)
        Can also be used to look into peoples bags at the airport
        Penetration: pass through air, low density solds, partially pass through bones
- Gamma rays
  - It is ionisong (lead to cancer) gamma rays have an even higher frequency than x-rays so can cause more damage and be absorbed by even deeper tissue
    - It can damage/ kill the cell in your body
      - Gamma rays can kill cancer cells and bacteria on food
        Can be used to sterlise medical instruments (kill microbes)
        Food can be sterilised in the same wa (killing microbes) - kepps food fresher for longer
        Used for cancer treatments (to kill cells/ tumor). To minimise damage to healthy cells the cancer is pinpointed then the radiation is moved around the body, this means healthy cells get less radiation
        Used for medical imaging. They are transmitted by skin, soft tissue and bone, so if they are produced inside a patient they can be detected outside
        Radiotracers (radioactive isotpes in the patients body) move around the body and emit gamma rays. Those gamma rays are detected by a gamma camera outside a patient to form an image. It can be used to see the inner workings of the body e.g. a cencerous tumor will take up more energy= bright spots or leaks in pipes like the intestine
        Penetration: high, it can pass through thehuman body
Imaging with electromagnetic waves
Electromagnetic waves and matter
Lenses
Light and colour
Wave experiments
- There are many different experiments you can carry out to demonstrate waves.
  - One way is to use an oscilloscop (to measure the speed of sound)
    - You have an oscilloscope with two microphones attatched, have a speaker making a sound. Then slowly move one microphone away from the other untill the wave patteren shows up the same on the oscilloscope, the distance between the mirophone is the wave length
  - You can also use a ripple tank
Ultrasounds
- It is a frequency greater than 20,000 Hz (human hearing)
  - It is useful as it has sall wave length so you can focus the beam
    - Uses of ultrasound: to attract the attention of specific animals, to scan a baby, cat alarms, anti-teenager alarms, dolphins and whales, bats (echo-location), sonar,, cleaning delicate instruments/jewellery,finding & treating kidney stones, purification of water
      - How ultrasound is used to do baby scans:
        1. Transmitter beams ultrasound waves into mother
        2.The waves reflect from the different boundaries
        3. The machine calculates the distances using time and velocity, and uses those distances to produce an image
        Sonar
        It is the vibration of particles
        You can use the same technique for sonar
        Sonar is used by submarines and fishermen
        They use echos which are reflected waves
  - There is also infresound which is a frequency lower than human hearing
Medical imaging (e.g. x-rays)
Human hearing: 20-20,000 Hertz (hz)
- e.g. 20Hz is when it vibrates 20 times a secon
  - Infersound: vibrations at a lower frequency than the human ear can hear
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