Light

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Light

• Jenny and Joel
• Townsville 2005

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• What are some different sources of light?
• What is light?
• What is colour?
• Have a look at different light sources through a
  diffraction grating
• The diffraction grating splits light into all the
  different colours that make it up
• White light is really all colours added together

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• Different colours of light are just waves of
  different energies

• The diffraction grating splits these up

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• A special source of light
• The Laser

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• L.A.S.E.R Light Amplification by Stimulated
  Emission of Radiation

• Light is bounced many times along a tube
• The gas is energised
• As the light bounces back and forth it gathers
  energy from the gas

• Lets see what it looks like!
• This light has three very special properties.

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Laser light is Monochromatic

• This means that it is all of a single colour
• Use your diffraction grating to check!
• The waves that make up the light all have the
  same length (the wavelength)

monochromatic
monochromatic
NOT monochromatic
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Laser light is Collimated

• The light from a laser is focused so that it
  doesnt spread out

This green laser light is collimated (the beams
dont spread out)
NOT collimated
NOT collimated
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Laser light is Coherent

• Light adds by the Principle of Superposition

If the waves line up, (in phase) you get a very
powerful wave
If the waves are exactly out of phase you get
no wave at all!
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• In a coherent beam, the light waves add up so
  that they are very powerful
• They dont cancel out anywhere

Coherent
NOT coherent (waves cancel at some points)
NOT coherent (waves cancel at some points)
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Lasers are very useful!

• Medicine and dentistry
• Very accurate and clean cutting

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Scientific Experiments

• Very well behaved, so extremely useful!

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• CD players
• Used to read the very fine grooves in the CD
• Party lights and laser pointers
• Heaps of other applications!

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• We can use a piece of hair to investigate the
  laser light
• What pattern do you expect to see when we put a
  hair across the beam?

Perhaps?

• Lets see!

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• Whats going on?
• Remember the Principle of Superposition?

If the waves line up, (in phase) you get a very
powerful wave
If the waves are exactly out of phase you get
no wave at all!
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• When you have the hair across the beam, this
  breaks it into two light sources

• The light from each side interferes
• Sometimes they add
• Sometimes they cancel
• This gives the light and dark pattern!

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• Your diffraction grating works in a similar way
• It has many many grooves in it, that each acts
  like a hair splitting the beam
• Interference from all the pieces of the beam
  splits up light into all its colours

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• Different colours of light are waves of different
  energies

• UV and Infrared are just colours that our eyes
  cant see
• Thats why UV is called black light (because
  you cant see it!)

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• UV light can be absorbed by things
• Some of the lights energy stays in the material
• Some energy is re-emitted as visible light (that
  we can see!)
• Check out the fluorescent minerals
• What other things glow?

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• Your clothes glow because your washing detergent
  contains fluorescent chemicals
• This makes your clothes seem very white in the
  sunlight (which has UV in it!)

• Your teeth glow because your toothpaste has
  fluorescent chemicals in it!
• Paper also has fluorescent chemicals in it!

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• UV light is dangerous
• It is much higher energy than visible light
• When your skin absorbs it, it damages your DNA
• This causes cancer!
• Thats why we wear sunscreen in the sun
• Sunlight has a lot of UV in it!
• Do you think our UV light is dangerous?

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• Our UV light is much weaker than the sun
• Its not dangerous at all!
• We can test this with UV sensitive beads
• These change colour in UV light
• How quickly do they change colour
• Under the UV light?
• Under the sun?
• Under sunlight through glass?
• What effect did the glass have?
• Does UV light pass easily through glass?

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Polarization

• Light waves can oscillate in different directions
• Just like waves in a rope

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• A polarizer only allows light oscillating in one
  direction to pass through it

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• If you have two polarizers that are crossed, no
  light will pass through!

Passes through
Rotate polarizer
Doesnt pass through (polarizers crossed)
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• Crossed polarizers dont allow light to pass
  through

• Test this with your polarizers

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• Why are polarisers useful?
• Where do we use polarisers?
• One place is Polaroid Sunglasses
• The lenses are also polarisers

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• Light reflected from a surface is polarized

• How could you use your polarizer to test this?
• Polaroid sunglasses remove glare

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• The blue light in the sky is polarized
• This is to do with the way that sunlight scatters
  off the atmosphere
• Is it polarized the same way all over the sky?
• Try looking
• Towards the sun (NOT AT THE SUN)
• Away from the sun
• Sideways to the sun
• Can you see a pattern?

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• What do you think should happen when you put
  three polarizers in a stack?
• Try it! What do you see?
• Have two crossed, then put one in between.
• Rotate the middle one
• What do you see?
• What is going on?

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• Polarized light can be split into components

• When the polarizers are not completely crossed,
  some light passes through
• This means that light can pass through three
  polarisers, even if the first and last are
  crossed!

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• Try putting the plexiglas between crossed
  polarizers
• Gently squeeze the plexiglas
• What do you see?

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• When under stress (being squeezed!) the plastic
  rotates the direction of polarization of light
• This means that light can pass through the
  stressed regions
• Engineers use this to determine stress patterns
  of buildings using plexiglas models

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• Try putting the sticky tape on plastic between
  crossed polarizers
• Rotate the plastic
• What do you see?
• When its pulled off the roll, the sticky tape
  molecules line up
• This makes them rotate the direction of
  polarisation of light
• Therefore light can pass through the layers of
  sticky tape
• The light rotates by different amounts depending
  on the thickness of tape
• Look at regions with several layers of tape
• It also depends on the colour of the light
• Thats why we see all the colours!

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• Try putting the mica between crossed polarizers
• Rotate the mica
• What do you see?
• Mica is a special type of rock that acts like the
  sticky tape
• It rotates the direction of polarization of the
  light
• It rotates by different amounts dependant upon
• The thickness of the mica
• The colour of the light

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• Try putting the calcite crystal over a page with
  text on it
• What do you see?
• Try putting a polarizer over the crystal
• Rotate the polarizer
• What do you see?

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• Calcite is a special crystal
• It is birefringent
• This means that it splits light into two parts
• Horizontally polarized
• Vertically polarized
• The two beams travel differently through the
  crystal
• The polarizer selects one at a time

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• Try putting a polarizer over the LCD screen of
  the calculator
• Rotate the polarizer
• What do you see?

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• LCD Liquid crystal Display
• What is a Liquid Crystal?

Molecules in a liquid
Molecules in a liquid crystal
Molecules in a solid crystal
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• The molecules rotate the direction of
  polarisation of light
• Electricity can make these will line up (no
  longer twisted)

Molecules in a twisted nematic liquid crystal
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• Putting a liquid crystal between crossed
  polarizers creates an LCD
• If electricity is put across a section, it goes
  black (light not rotated)

LCD display with no field applied
LCD display with a field applied
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Optical Fibres

• Optical fibres are used to transmit light long
  distances
• Light travels along the fibre
• It bounces along the inside
• This happens by Total Internal Reflection