Light and Color

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Light and Color
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Light

• Early Concepts
• Socrates-light comes from your eye to the object
• You see something when your eyes fall on it
• Like streamers from a pompom
• Pythagoras and Newton-light travels to eye as
  particles
• Huygens-light is a wave
• Einstein-photoelectric effect
• Particles of energy-photons
• Travel in a wave pattern

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Speed of Light

• First suspected by Roemer in observations of Io
• First measured by Huygens from same observation
• More accurately measured by Michelson
• Used spinning octagonal mirrors and a mirror far
  away
• Adjusted spinning speed to calculate speed of
  light
• Value 3.0 x 10 8 m/s
• Used to measure large distances, light year
  light travels in one earth year

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Electromagnetic Waves

• Electromagnetic Spectrum
• Energy moving at speed of light
• Vary by f and l only
• Light is only a small part
• Below light is infrared, above is ultraviolet

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Transparent Materials

• Light causes vibrations in matter similar to
  sound but of the electrons
• Visible light, with lower frequency, does not
  cause resonance of electrons, and passes from
  atom to atom and through the glass by absorption
  and reemission
• Light travels slower through denser media because
  of the delay between absorption and reemission

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Transparent Materials

• Glass electrons have natural frequency in UV, so
  UV light is absorbed and not transmitted
• Infrared vibrates the glass structure and again
  does not pass through due to absorption

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Opaque Materials

• Non metallic materials change light to heat
  energy
• Metallic materials reemit the light in reflection
  which makes metals shiny
• Atmosphere is mostly opaque to ultraviolet due to
  ozone layer, transparent to light and infrared

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Shadows

• Light beams are made of bundles of rays
• Rays can be blocked to make shadows
• Sharp shadows come when small light is near or
  large light far from an object
• Blurry shadows have total shadow part (umbra) and
  partial shadow (penumbra)
• Eclipse of sun is good example of this

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Shadows

• Shadows also occur when light is bent when
  passing a transparent material
• This causes the twinkling of stars

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Polarization

• Wave nature of light is transverse, vibrating
  across line of travel
• Light can vibrate at any plane angle to its
  direction of travel this happens from electrons
  vibrating in different directions
• Regular light sources are non-polarized-many
  electrons vibrating
• Filters can allow vibration in one direction
• Contain a polarizing axis
• Like light coming in through a fence-openings
  allow it to pass through

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Polarization

• Light reflects in polarized fashion
• Light not in the same vibration as surface
  direction is absorbed
• Light same direction is reflected- glare
• Horizontal glare is eliminated by vertical
  filters
• 3D viewing uses polarizing filters
• One over each of two projectors forms the image
• Viewer uses glasses with polarizing filters to
  view the scene

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Color

• Color Spectrum
• First studied by Newton
• Used prism to separate colors
• Order- red, orange, yellow, green, blue, violet
• Sunlight and incandescent are white light
• White not a color
• Black is the absence of light-absorbs light

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Color by Reflection

• Color reflects by setting electrons to vibrating
• Light not absorbed in resonance is reemitted as
  color if object is opaque
• Color reflected is the color perceived
• Reflected color is usually of many frequencies
  combined
• Color reflected depends on light causing it
• Candle light is deficient in blue
• Incandescent enhances reds
• Fluorescent enhances blues
• Sunlight is best for distinguishing
• color

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Color by Transmission

• Transparent objects absorb color as light passes
• Pigment allows viewed color through only
• Sunlight strongest in yellow -green frequency

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Mixing Colored Light

• Transmitted light mixes differently from paint
• Red, Green, and blue light mix
• red and blue magenta red and green yellow
  
• green and blue cyan All three white
• Color television uses this principle
• Complementary light colors when added white
• red and cyan yellow and blue
• Subtraction can make the complement
• Absorbs blue-reflects yellow
• Absorbs red-appears cyan

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Mixing Pigments

• Pigments create color by absorbing and reflecting
  
• Mixing causes selective subtraction
• Pigments used in printing are yellow and cyan,
  magenta -produce all others by subtractionColor
  Box Applet

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Natural Colors

• Sky is blue as light passes air molecules, it is
  absorbed and reemitted in many directions
• Reemitted light is said to be scattered
• Clouds are white because they scatter all light
  equally

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Natural Colors

• Sunset is red
• Red is scattered least by atmosphere
• Light passing through near sunrise or sunset
  passes through most atmosphere
• Red gets through most atmosphere so sun is red
  near ends of the day

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Natural Colors

• Water is greenish blue (cyan)
• Water absorbs infrared and visible-red
• Color complementary to red is cyan-colored of
  water
• Colors depend on reflected frequencies of
  molecules, and absorbed frequencies by molecules

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Atomic Spectra

• Atoms emit light by their own vibrations of
  electrons
• Atomic spectra are discovered with spectroscope
• Arrangement of lenses and prism to magnify colors
• Spectra are collections of bright lines which
  identify elements

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Homework!!

• Complete the Worksheet Assigned