Topic 5 - Light and the electromagnetic spectrum

• opaque object don't transmit light
• colour depends on wavelengths of the light that are most reflected.
• red aple absorb all other wavelength and only reflect red light
• black object absorb a;; wavelength of visible light
• white object reflect all wavelength of visible light equally
• staring from red, it has the longest wavelength and lower frequency with puple to end with the shortest wavelength and highest frequency
• transparant (see-throught ) or translucent (partially see through ) objects transmit light
• white light made up of all colours

  whe it is refracted it cause dipersion as different wavelength is slowdown by different amounts
• Colour filter trabsmit certain colour (wavelengths ) and absorb the rest

  if no light is trabsmitted, the object will appear black
• For colour, frequency and wavelength determind the colour and for sound, freqeuncy determind the pitch
• real image: can be captured by a screen and light ray actually meet at the places where image seem to be
• A virtual image: cannot be captured by a screen and light ray don't actually come together at the point where image seem to be
• Converging lens can produce real or virtual image

  • principal focus : point where rays that are parallel to axis come together on the axis
  • focal length: distance between centre of lens and principal focus
• Diverging lens: always produce virtual image

  • focal length: same side as principal focus
  • principal focus: : point where rays hitting lens parallel to axis appear to come from
• How to increase the power of lenses:

  • more curved
  • converging = positive power
  • diverging = negative power
• More powerful the lens:

  • more stronger it converges/ diverges
  • shorter the focal length
• elctromagnetic spectrum:

  radiowave, microwave, infrared, visible light, ultraviolet, X-ray, Gamma ray

  • increasing frequency, decreasing wavelength -->
• Our eye can only detect visible light
• EM wave can:

  • transfer energy from source to absorber
  • travel at the same speed in a vaccum
• changes in atom and nuclei can generate EM wave over a large frequency range
• Changes in atoms and nucei can be caused by absorption of EM wave over a large frequency range
• Radio wave:

  • statellite transmission: transmitter send signal to statellite and it give the signal back to a satellite dish --> longwave
  • Communication: bluetooth --> shortwave
  • broadcasting: TV, bluetooth --> short wave
  • long wave = bend around the curved surface of earth = radio signal can be recieved even the receiver isn't in the sight
  • short wave = must be in direct sight
• Microwave:

  • microwave ovens: microwave absorbed by water molecules in food, transfer energy to them = heat up = cook
  • statellite transmission
  • communication
• Visible light:

  • illumination
  • photography
• Infrared radiation:

  • electric heater: wire heats up when current pass through it = emits a lot of infrared radiation = thermal energy store = heat up
  • security system: infrared sensor = change in infrared radiation =alarm
  • thermal iimaging: different colour= different amount of IR radiation = escape or hide in dark
  • food can be cooked using IR radiation
  • TV remote control: receivor must be in line of sight of emitter
  • optical fibre: use single wavelength to prevent dispersion = info lost + total internal reflection
• UV wave:

  • Fluorescent lamp: : use UV to emit visible light, good when light is needed for long period
  • Security pen: mark property, invisible, but glow under UV light
  • detecting forged bank notes: special marking
  • sterilise water: kills bacteria and safe to drink
• X-ray

  • medical X-ray: tranmitted by flesh but absorbed by denser object like bones = internal structure pf body = broken bones
  • airport security scanners: detect hidden object that can't be detected by metal detectors
• Gamma ray:

  • sterilise food and medical equipment: kill microbes + safe
  • detect and treat cancer: tracer = detect, radiation = kill
• Danger of exposure :

  • as frequency increase, danger increases
  • microwave ( not ionising) : heat up cell
  • infrared: burn skin
  • ultraviolet: cause damage to cells on surface of skin = skin cancer + damage eyes = blindness
  • X-ray + Gamma ray : cause cell damage + mutations = cancer
• The rate of absorption and emission determinds the temperature of an object

  • average power emitted by an object > average power absorb = cool down
  • average power emitted by an object < average power absorb = heat up
  • average power emitt by an object = average power absorb = constant temperature
• Temperature of an object example

  hot cup of tea

  • it emitts more average power ( as it is hot) than it absorb
  • Therefore it will cool down eventually
• Temperature and radiation

  • All objects are continually emitting (radiating) and absorbing EM radiation over a range of wavelength
  • The distribution and intensity of wavelength only depends on it's temperature
• The difference in intensity that each wavelength emit differ

  intensity increase more rapidly for shorter wavelength than longer wavelength
• Why does the curve shift to the left and peak higher over time

  • As temperature of an object increase, the intensity of every emitted wavelength increase
  • shift to the left as shorter wavelength emmits higher intensity rapidly
  • peak higher due to the increase in intensity

  
• Radiation and Earth's temperature

  • Daytime: absorb > emit = local temperature increase
  • Nightime: emit > absorb = local temperature decrease
  • Overall temperature of earth remain fairly constant as there will always be part of earth under the sunlight and some in the dark
• Radiation and earth temperature (absorption and emission)

  Atmosphere, cloud and earth surface are the main source of absorption, emission and reflection

  A) emitted

  B) atmosphere

  C) clouds

  D) earth surface

  E) reflected

  F) absorbed

  
• The difference in the absorption and emission of colours

  • black surface are good emitter and good absorber of infra-red
  • Light,shiny surface are poor absorbers and poor emitters of infra - red ( good reflectors)
• Investigate absorption by melting wax

  • independent variable: different colour on the other side of metal plate
  • dependent variable: which ball bearing fall first / faster rise in temperature
  • controlled variable: same distance apart, identical metal plates
  • Ball bearing on the black plate will fall first as the black surface is a better absorber = energy transferred to the thermal energy store of wax = melt = fall

  
• Investigate the emission of radiation

  • independent variable: different surface
  • dependdent variable: rate of decrease in water temperature
  • Control variable: volume of water
  • Boil water and fill each test tube with the same volume
  • Measure the temperature of water at regular intervals
  • better emitter = temperature drop quicker
  • black is better emitter than white. A matte surface better emitter than shiny