LIGHT

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LIGHT
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WHAT IS LIGHT?

• All frequencies or wavelengths of
  electromagnetic radiation
• Seven bands of electromagnetic waves, based upon
  differences in frequency and wavelength.
• A continuous spectrum
• A form of energy.

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WHAT CAUSES Electromagnetic waves (or LIGHT)?

• Vibrating electrically charged particles
• A changing electric field sets up a changing
  magnetic field, which sets up a changing electric
  field, and so on.

Electromagnetic wave (a combination of electric
and magnetic fields
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Electromagnetic Wave
http//www.nrc-cnrc.gc.ca/obj/inms-ienm/images/res
earch_images/optical_comb/COMBFIG1.gif
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Electromagnetic Waves
Light is a portion of the family of
electromagnetic waves that includes radio waves,
microwaves, and X-rays. The range of
electromagnetic waves is called the
electromagnetic spectrum.
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The Electromagnetic Spectrum

• Non-mechanical waves are produced by vibrating
  electric charges (changing electric field
  produces a changing magnetic field and
  vice-versa)
• Spectrum means a continuous series of wavelengths
  and frequencies
• Divided into 7 characteristic portions-differ by
  wavelength
• As frequency increases, wavelength decreases
• Light shows a dual nature
• Modeled as Wave (reflects, refracts, diffracts,
  interferes)
• Modeled as Particles a stream of photons
• As the frequency increases, so does the energy of
  the photon
• All objects emit electromagnetic radiation
  (higher the temperature, the higher the frequency
  of the electromagnetic spectrum)

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• http//phet.colorado.edu/en/simulation/photoelectr
  ic

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Visible Light

• Smallest portion of electromagnetic spectrum
• Different frequencies represent different colors
• White light contains all frequencies of visible
  light (all colors)
• ROYGBVfrom longest to shortest wavelength or
  lowest to highest frequency, red light has lower
  frequency and is less energetic than violet light
• White light is the presence of all color
  (reflection) black light is the absence of all
  color (absorption).
• The color we see represents only those
  frequencies of visible light that are reflected
  by the object
• Primary colors of light (Red Blue Green
  White)
• Red Blue (magenta) Blue Green (cyan) Red
  Green (yellow)
• Photoelectric effect visible light photons cause
  electrons to move in solar cells, producing
  electricity.
• The cone cells in the retina are light-sensitive
  and respond to the different frequencies of color
  
• The pupil will change size to adjust for
  intensity (brightness) of light

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Graphical Representation of Wavelength and Light
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HOW DO WE MODEL LIGHT?

• TWO MODELS
• Wave Model
• Particle Model
• Physicists recognize the dual nature of light
  (light travels like a wave, but hits like a
  particle).
• Photon a massless particle of light (carries
  energygreater the frequency, the greater the
  photon energy)

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LIGHT AS A WAVE AND A PARTICLE

• Light behaves like a wave because light reflects,
  refracts, diffracts and interferes.
• Light also behaves like a particle, traveling in
  a stream photons.
• Photons are discrete packets of light energy.
• Evidence for the particle model is the
  photoelectric effect.

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ENERGY OF A PHOTON

• Photon energy is proportional to the frequency of
  light. The higher the frequency, the higher the
  photon energy.
• For example, radio wave photons carry less energy
  than do ultraviolet light photons.

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Light and Transparent Materials
A light wave incident upon a pane of glass sets
up vibrations in the atoms. Because of the time
delay between absorptions and reemissions, the
average speed of light in glass is less than c.
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INTENSITY OF LIGHT

• The amount of light illuminating a surface or
  passing through space is called the intensity
  (brightness).
• Intensity relates to the number of photons, not
  the energy of a photon.

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INTENSITY OF LIGHT

• An increase in the number of waves or photons (or
  the amount of light)
• Brightness of light means intensity.
• Not related to frequency or the energy per photon!

Same frequencies of light, just different
intensities!
100 Watts
40 Watts
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SPEED OF LIGHT

• Nothing in the Universe travels faster than the
  speed of light.
• Speed
• 3 x 108 m/s
• 300,000 km/s
• 186,000 mi/s

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WHAT AFFECTS THE SPEED OF LIGHT?

• Speed of light is affected by the type of medium
  it travels through.
• In general, light travels fastest through a
  vacuum, then gases, liquids and solids.
• This is opposite of mechanical waves. Sound
  waves travel fastest through solids, not gases.

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Photoelectric Effect the ejection of electrons
from certain metals when exposed to certain
frequencies of light.
Electrons emitted
Electromagnetic radiation (visible light range)
Photovolatic cell (solar cell)
Produces electron flow (electricity)
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Energy transformations Observed with photovoltaic
cell, motor and propeller

• Visible light absorbed by photvoltaic cell (light
  energy to electrical energy)
• Electric motor causes propeller to spin
  (electrical energy to mechanical energy)

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WHAT IS LIGHT?

• All forms of electromagnetic radiation
• http//www.colorado.edu/physics/2000/waves_particl
  es/
• http//didaktik.physik.uni-wuerzburg.de/pkrahmer/
  ntnujava/emWave/emWave.html
• http//imagers.gsfc.nasa.gov/ems/waves3.html
• http//ihome.cuhk.edu.hk/s016969/physproj/index.h
  tm
• http//imagine.gsfc.nasa.gov/docs/science/know_l1/
  emspectrum.html