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physics: waves (transverse & longitudinal waves (when waves travel…

- - - - in a spring: rarefaction - coils spread out
        compression: coils together
- - - - the microwaves penetrate up to a few centimetres into the food before being absorbed & transferring the energy they are carrying to the water molecules in the food, causing the water to heat up.
        
        the water molecules then transfer this energy to the rest of the molecules in the food by heating - quickly cooks the food.
  - - - infrared cameras can be used to detect infrared radiation & monitor temperature. the camera detects the IR radiation & turns it into an electrical signal which is displayed on a screen as a picture. the hotter the object is, the brighter it appears.
        e.g. energy transfer from a house's thermal energy store can be detected using infrared cameras
        
        absorbing IR radiation causes objects to get hotter. food can be cooked using IR radiation - the temperature of the food increases when it absorbs IR radiation e.g. from a toaster's heating element.
        
        electric heaters can heat a room as they contain a long piece of wire that heats up when a current flows through it. the wire then emits lots of IR radiation (& a little visible light, the wire glows).
        
        the emitted IR radiation is absorbed by objects & the air in the room - energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperatures to increase.
  - - - satellite TV - signal from a transmitter is transmitted into space where it is picked up by the satellite receiver dish orbiting thousands of kilometres above the Earth.
        the satellite transmits the signal back to Earth in a different direction where it is received by a satellite dish on the ground - slight time delay between signal being sent & received because of the long distance the signal has to travel.
  - - - they work because of reflection. the light rays are bounced back & forth until they reach the end of the fibre.
        
        visible light used.
        
        light is not easily absorbed or scattered as it travels along a fibre.
  - - - security pens can be used to mark property with your name. under UV light the ink will glow (fluoresce) but is invisible otherwise - help police to identify your property if it is stolen.
  - - - the brighter bits are where fewer X-rays get through - this is a negative image - the plate starts off all white.
- - - - when light is reflected by a rough surface, the surface appears matte (not shiny) & you don't get a clear reflection of objects
- - - - when signal generator & vibration transducer turned on, string starts to vibrate.
        
        adjust frequency of signal generator until there is a clear wave on the string. frequency needed depends on length of string between pulley & transducer & the masses used.
        
        measure wavelength of waves accurately by measuring the lengths of e.g. 4 or 5 half-wavelengths (as many as poss.) at once then divide to get mean half-wavelength, double this to get full wavelength.
        
        frequency of wave is what signal generator is set to.
        
        find speed with v = fλ
        
        suitable for investigating waves on a string as it's easy to see & measure wavelength.
  - - - set up oscilloscope so detected waves at each microphone are shown as separate waves.
        
        start with both microphones next to speaker then slowly move one away until the two waves are aligned on the display but have moved exactly one wavelength apart.
        
        measure distance between microphones to find one wavelength (λ)
        
        use v = fλ to find speed of the sound waves passing through the air, the frequency is what the signal generator was set to (around 1kHz)
        
        speed of sound in air 330 m/s - results should roughly agree.
- - - - wavelength changes when it is refracted but frequency stays the same.
        
        if wave is travelling along the normal it will change speed but is not refracted
        
        wave fronts (lines) closer together shows a change in wavelength & so change in velocity.
  - - - they form a continuous spectrum over a range of frequencies. grouped based on wavelength & frequency
        
        there is such a large range of frequencies because EM waves are generated by a variety of changes in atoms & their nuclei e.g. changes in the nucleus of an atom creates gamma rays & explains why atoms can absorb a range of frequencies - each one causes a different change.
- - - - place a transparent rectangular block on a piece of paper & trace around it. use a ray box or laser to shine a ray of light at the middle of one side of the block.
        
        trace the incident ray & mark where the light ray emerges on the other side of the block & with a straight line, join up the incident ray w/ the emerging point to show the path of the refracted ray through the block.
        
        draw the normal at the point where the light ray entered the block. use a protractor to measure the angle between the incident ray & the normal (the angle of incidence,I) & the angle between the refracted ray & the normal (the angle of refraction, R)
        
        repeat using rectangular blocks made from different materials, keeping the incident angle the same throughout.
        
        angle of refraction changes for different materials due to their different optical densities
  - - - smooth surfaces like mirrors give clear reflections (the reflected ray is as thin & bright as the incident ray).
        
        rough surfaces like paper cause diffuse reflection which causes the reflected beam to be wider & dimmer (or not observable at all)
        
        angle of incidence = angle of reflection
- - - - colours can mix together to make other colours. the only colours that can't be made by mixing are the primary colours: red, green & blue.
        when all of these colours are put together, white light is created.
  - - - some wavelengths of light may be absorbed or reflected by transparent & translucent objects - its colour is related to the wavelengths transmitted & reflected by it.
  - - - if the object was e.g. red (of any colour not made from blue light), the object would appear black when viewed through a blue filter. All of the light reflected by the object is absorbed by the filter.
      - filters that aren't for primary colours let through both the wavelengths of light for that colour AND the wavelengths of the primary colours that can be added together to make that colour.
- - - - a paper diaphragm in a speaker vibrates back & forth, causing the surrounding air to vibrate, creating compressions (particles closer together) & rarefractions (particles further apart) - a sound wave is created.
        
        when the sound wave hits a solid object, the air particles hitting the object (the pressure) causes the particles in the solid to vibrate. these particles hit the next particles in line & so on - passing the sound wave through the object as a series of vibrations.
  - - - human hearing is limited by the size & shape of the ear drum as well as the structure of the parts within the ear that vibrate to transfer the energy from the sound waves.
- - - - short-wave radio signals (wavelengths 10m-100m) can also be received at long distances from the transmitter as they are reflected from the ionosphere - an electrically charged layer in the Earth's upper atmosphere.
        
        Bluetooth uses short-wave radio waves to send data over short distances between devices without wires (e.g. wireless headsets so you can use your phone while driving a car).
        
        medium-wave signals can also reflect from the ionosphere depending on atmospheric conditions & the time of day.
        
        the radio waves used for TV and FM radio transmissions have very short wavelengths.
        to get reception, you must be in direct sight of the transmitter - the signal doesn't bend or travel far through buildings.
- - - - UV radiation damages surface cells which can lead to sunburn & cause skin to age prematurely. more serious effects are blindness & an increased risk of skin cancer.
      - X-rays & gamma rays are types of ionising radiation (they carry enough energy to knock electrons off of atoms) which can cause gene mutation or cell destruction & cancer.
  - - - the risk depends on the total amount of radiation absorbed & how harmful the type of radiation is.
        1000 mSv = 1 Sv
        
        a CT scan uses X-rays & a computer to build up a picture of the inside of a patient's body. risks can be different for different parts of the body - if a patient has a CT scan on their chest, they are four times more likely to suffer damage to their genes (their added risk of harm four times higher) than if they had a head scan.
        head dose: 2.0 (mSv)
        chest dose: 8.0 mSv
- - - - objects at a constant temperature emit infrared radiation at the same rate they are absorbing it.
        :arrow_right_hook: some colours & surfaces absorb & emit radiation better than others e.g. a black surface is better at absorbing & emitting radiation than a white one & a matt surface is better at emitting & absorbing than a shiny one .
  - - - place an empty Leslie cube on a heat-proof mat.
        
        boil water in a kettle & fill the Leslie cube with boiling water.
        
        wait for the cube to warm up then hold a thermometer against each of the four vertical faces of the cube - all four faces should be the same temperature.
        
        hold an infrared detector at a set distance (e.g. 10cm) from one of the cube's vertical faces & record the amount of IR radiation detected.
        
        repeat for each vertical face - position detector at same distance each time.
        
        more IR radiation should be detected from the black surface than the white one & more from the matt surfaces than the shiny ones.
        
        repeat the experiment to make sure the results are repeatable.
        
        don't move the cube when it is full of boiling water as you might burn your hands + be careful when carrying the kettle.
      - faces:
        
        matt black paint
        
        matt white paint
        
        shiny metal
        
        dull metal
      - investigate how absorption depends on the surface by:
        
        sticking ball bearings to the back of two different surfaces with wax & see which one falls off first when the surfaces are placed equal distances from a Bunsen burner. (if doesn't absorb much, wax will melt causing ball to fall).
- - - - the intensity & distribution of the wavelengths emitted by an object depends on the object's temperature.
        intensity is the power per unit area i.e. how much energy is transferred to a given area in a certain amount of time.
        
        as the temperature of an object increases, the intensity of every emitted wavelength increases but the intensity increases more rapidly for shorter wavelengths which causes the peak wavelength (highest intensity) to decrease.
  - - - overall, the temperature of the Earth stays fairly constant.
      - changes to the atmosphere can cause a change to the Earth's overall temperature - if the atmosphere starts to absorb more radiation without emitting the same amount, the overall temperature will increase until absorption & emission are equal again (global warming).
- - - - when seismic waves reach a boundary between different layers of material (which have different properties like density) inside the Earth, some waves are absorbed & some are refracted.
        
        if the waves are refracted they change speed gradually, resulting in a curved path but when the properties change suddenly, the wave speed changes abruptly & the path has a kink.
- - - - the principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet.
      - an incident ray parallel to the axis refracts through the lens & passes through the principal focus on the other side.
        
        an incident ray passing through the principal focus refracts through the lens & travels parallel to the axis.
        
        an incident ray passing through the centre of the lens carries on in the same direction.
    - - the principal focus of a concave lens is the point where rays hitting the lens parallel to the axis appear to all come from - they can be traced back until they all appear to meet up at the point behind the lens.
      - an incident ray parallel to the axis refracts through the lens & travels in line with the principal focus (so it appears to have come from the principal focus).
        
        an incident ray passing through the lens towards the principal focus refracts through the lens & travels parallel to the axis.
        
        an incident ray passing through the centre of the lens carries on in the same direction.