P6

• When waves travel through a medium, the particles of the medium oscillate and transfer energy between each other. Overall the particles stay in the same place and only energy is transferred
• • amplitude of a wave - maximum displacement of a point on the wave from it's undisturbed position
  • wavelength - distance between the same point on two adjacent waves
  • frequency - number of complete waves passing a certain point per second
• Transverse Waves

  • oscillations are perpendicular to the direction of energy transfer
  • examples: ripples and waves in water, all EM waves
• Longitudinal Waves

  • oscillations are parallel to the direction of energy transfer
  • examples: sound waves, shock waves
• When a wave meets a boundary between two materials it can be

  • absorbed
  • transmitted
  • reflected
• EM Waves

  • transverse waves
  • all travel at the same speed through air or a vacuum
• Refraction - changing direction at a boundary

  • the higher the density of a material, the slower a wave travels through it
  • if a wave crosses a boundary and slows down it will bend towards the normal
  • if a wave crosses into a material and speeds up it will bend away from the normal
  • wavelength changes when refracted but frequency stays the same
• Optical Density - a measure of how quickly light can travel through it

  -> the higher the optical density, the slower light waves travels through it
• • low frequency waves don't transfer much energy and so mostly pass through soft tissue without being absorbed
  • high frequency waves transfer lots of energy so can cause lots of damage
• UV radiation damages surface cells, which can lead to sunburn and cause skin to age prematurely. Could also increase risk of skin cancer
• X-rays and gamma rays are types of ionising radiation. This can cause gene mutation, cell destruction and cancer
• Radiation dose (measured in Sieverts, Sv) is a measure of the risk of harm from the body being exposed to radiation
• Specular reflection happens when a wave is reflected in a single direction by a smooth surface
• Diffuse reflection is when a wave is reflected by a rough surface and the reflected rays are scattered in lots of different directions

  Happens because the normal is different for each incoming ray
• You can produce radio waves using an alternating current in an electrical circuit.

  The object in which charges oscillate to create the radio waves is called a transmitter
• Radio Waves

  • when transmitted radio waves reach a receiver, the radio waves are absorbed
  • the energy carried by the waves is transferred to the electrons in the material of the receiver
  • this energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, it generates an alternating current
  • this current has the same frequency as the radio wave that generated it
• Radio Waves - Communication

  • long wavelengths can diffract (bend) around the curved surface of the earth.
  • long-wave radio wavelengths can also diffract around hills, into tunnels etc
  • this makes it possible for radio signals to be received even if the receiver isn't in line of the sight of the transmitter
• Radio Waves - Communication

  • short-wave radio signals can be received at long distances from the transmitter
  • this is because 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
• Radio Waves - Communication

  • medium-wave signals can also reflect from the ionosphere, depending on the atmospheric conditions and the time of day
• Radio Waves - Communication

  • radio waves used for TV and FM radio transmitters have very short wavelengths
  • to get reception, you must be in direct sight of the transmitter as it doesn't bend or travel far through buildings
• Microwaves - Satellites

  • communication to and from satellites uses microwaves
  • it's best to use microwaves which can pass easily through the earths watery atmosphere
• Microwaves - Satellites

  • for satellite tv, the signal from a transmitter is transmitted into space
  • where it's picked up by the satellite receiver dish orbiting thousands of kilometres above earth
  • the satellite transmits the signal back to earth in a different direction
  • where it's received by a satellite dish on the ground
• Microwaves - Microwave Ovens

  • the microwaves are absorbed by water molecules in food
  • the microwaves penetrate up to a few cm into the food before being absorbed and transferring the energy they're 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
• Infrared radiation is given out by all hot objects - the hotter the object, the more IR radiation it gives out
• Infrared Radiation - Cameras

  • infrared cameras can be used to detect infrared radiation and monitor temperature
  • the camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture
  • the hotter the object, the brighter it appears
• Infrared Radiation - Cooking Food

  • 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
• Infrared Radiation - Electrical Heaters

  • contain a long piece of wire that heats up when a current flows through it
  • this wire then emits lots of infrared radiation
  • the emitted IR radiation is absorbed by objects and the air in the room
  • energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperature to rise
• Visible Light - Fibre Optic Cables

  • optical fibres are thin glass or plastic fibres that can carry data over long distances as pulses of visible light
  • they work because of reflection
  • the light rays are bounced back and fourth until they reach the end of the fibre
  • visible light is used in optical fibres
  • light is not easily absorbed or scattered as it travels along a fibre
• UV Radiation - Energy Efficient Lamps

  • fluorescent lights generate UV radiation, which is absorbed and re-emitted as visible light by a layer of a compound called a phosphor on the inside of the bulb
  • they're energy efficient so they're good to use when light is needed for long periods
• fluorescence is a property of certain chemicals, where ultra-violet radiation is absorbed and visible light is emitted - thats why fluorescent colours look so bright
• UV Radiation - Suntan

  • produced from the sun, and exposure to it is what gives people a suntan
  • when it's not sunny, some people go to tanning salons where UV lamps are used to give them an artificial suntan
  • however, overexposure to UV radiation can be dangerous
• X-Rays - X-Ray Photographs

  • x-rays pass easily through flesh but not so easily through denser materials like bones or metal
  • it's the amount of radiation absorbed that gives you an X-ray image
• X-Rays and Gamma Rays - Radiotherapy

  • can be used to treat people with cancer
  • high doses of these rays kill all living cells
  • they can be carefully directed towards cancer cells to avoid killing too many healthy cells
• Gamma Radiation - Medical Tracer

  • gamma-emitting source is injected into the patient
  • it's progress is followed around the body
  • it can pass out through the body to be detected
• Lenses form images by refracting light and changing it's direction
• Convex lenses cause rays of light parallel to the axis to be brought together at the principal focus
• Concave lens causes parallel rays of light to diverge (spread out)
• The image produced by a convex lens can be real or virtual

  the image produced from a concave lens is always virtual
• Lens Diagrams - Describing An Image

  • if image is larger than object: magnified
  • if image is smaller than object: diminished
  • if the image is the same way up as object: upright
  • if the image is upside down: inverted
  • if image is on same side as object: virtual
  • if image is on opposite side to object: real
• Each colour within the visible light spectrum has it's own narrow band of wavelength and frequency