Chapter 27- Light

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Chapter 27- Light
Light is the only thing we see!
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Early Concepts of Light

• Light consists of particles!
• Socrates Plato believed that vision resulted
  from streamers emitted by the eye making contact
  with an object.
• Light seems to move in a straight line!

• Light consists of waves!
• Empedocles Christian Huygens argued that light
  travels in waves.
• Sometimes light does spreads out!
• 1905- Einsteins theory is that light consists of
  particles of electromagnetic energy.

Scientists now agree that light is part particle
and part wave!
3
The Speed of Light
The Speed of Light 300,000 km/s

• Roemer found that light coming from Jupiters
  moon Io takes a longer time to reach earth at
  position D than at position A. The extra distance
  that the light travels divided by the extra time
  it takes gives the speed of light.

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The Speed of Light
The Speed of Light 300,000 km/s

• Michelson used a mirror arrangement to measure
  the speed of light. The speed of the rotation of
  the mirror was adjusted so that the round trip of
  the light was the same as the rotation of the
  mirror and would reflect the light into the
  eyepiece. He divided the 70-m distance by the
  time and found the speed of light.

5
Electromagnetic Waves

• Light is energy that travels in electromagnetic
  waves within a certain range of frequencies.
• Electromagnetic Spectrum - the range of
  electromagnetic waves.
• The lowest frequency of visible light is red and
  highest is violet.
• Infrared - electromagnetic waves of frequencies
  lower than the red light.
• Ultraviolet - electromagnetic waves of
  frequencies higher than violet.

6
Light and Transparent Materials

• Light waves can force charged particles in
  materials into vibration.
• How the material responds to light depends on the
  frequency of light the frequency of electrons
  in the material.
• Light passes through materials whose atoms absorb
  the energy and immediately reemit it as light.
• Transparent materials glass water.
• Ultraviolet Infrared light cannot pass through
  glass. Visible light can pass through glass.
• Light travels at different speeds through
  different materials! Vacuum 300,000 km/s or c,
  water 0.75c, glass 0.67c, diamond 0.40c.
  When light emerges from materials into air it
  travels at its original speed, c.

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Opaque Materials

• Light doesnt pass through a material when the
  energy is changed to random kinetic energy of the
  atoms. Opaque materials Wood, Stone, People,
  etc.
• Metals are also opaque. When light shines on
  metal, free electrons vibrate and reemit the
  light as a reflection.
• Our atmosphere is transparent to visible light
  but opaque to ultraviolet waves. Ultraviolet
  light causes sunburns. Clouds are semitransparent
  to uv and sand water reflect uv waves.

8
Shadows

• Ray thin beam of light.
• Shadow shaded region that results where light
  rays cant reach.
• Umbra total shadow.
• Penumbra partial shadow due to some light being
  blocked and other light filling in.
• Solar and lunar eclipses!
• Shadows occur in water. Light travels at
  different speeds in warm and cool water. The
  difference bends light causing shadows.

9
Polarization

• Light waves are transverse waves!
• A polarized wave is one that travels back in
  forth in one direction. (Vertically
  Horizontally polarized waves)
• The sun, a candle flame or an incandescent lamp
  emits light that is not polarized. This light
  vibrates in random directions.
• Polarization The aligning of vibrations in a
  transverse wave, usually by filtering out waves
  of other directions.
• Polarizing filters transmit components of
  non-polarized light that are parallel to the
  polarization axis, and block components vibrating
  at right angles to the polarization axis.

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Polarized Light 3-D Viewing

• Vision in 3 dimensions is due to both eyes
  viewing a scene from a different angle.
• 3-D movies stereo grams uses this technique!