Light and the Electromagnetic Spectrum

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Light and the Electromagnetic Spectrum
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How We See the Universe

• We see the Universe in visible light
• Visible light between 700nm (red) and 400 nm
  (violet)
• The spectrum of all EM radiation
• gamma rays
• X-rays
• Ultraviolet (UV)
• Visible light
• Infrared (IR)
• Microwaves
• Radio

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Electromagnetic Waves

• EM Waves are a response to changes in electrical
  and/or magnetic fields elsewhere. (accelerated
  charges)
• EM Waves are transverse waves (like ripples on a
  pond)
• EM waves do NOT need a medium to travel through
• ALL EM Waves travel at the speed of light
• (c 3 x 108 m/s)

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The Electromagnetic Spectrum

• High Energy Low energy
• High Frequency Low frequency
• Short wavelengths Long wavelengths

What we see (visible light) VIB G YOR
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Visible light Spectrum

• Isaac Newton showed that ordinary sunlight could
  be split into many colors with a prism
  (dispersion)
• ROY G BIV
• (red, orange, yellow, green, blue, indigo,
  violet)
• Each color corresponds to light of a specific
  wavelength with a specific frequency

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Example EM Wave
Wavelength, ?, is length from crest to
crest Frequency, f is the number of wave crests
per second that pass a given point speed
v f ? ALL emag. Waves travel at the speed of
light ( 3 x 108 m/s)
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Example

• A yellow light wave has a frequency of 5 x
  1014 Hz. What is the wavelength of this yellow
  light?
• (all emag waves travel at speed of light)
• v f ?
• 3x108 m/s (5x1014 Hz) ?
• (divide by 5x1014 Hz)
• ? 6 x 10-7 m

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Wave-Particle duality

• Light behaves like a wave (proof double slit
  interference pattern)
• Light behaves like a particle (proof
  photoelectric effect)
• Light is BOTH a particle and a wave!

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Photoelectric effect

• Light shines on a metal and kicks out electrons.
• We get more energetic electrons byshining a
  higher frequency light on it (blue?violet)
• Brighter light ejects more electrons but not with
  higher energies.
• Uses???? (solar powered calculatorsor solar
  powered anything)
• E hf (h 6.63 x 10-34 J-s )
• electron

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Brightness

• Flux (rate of energy per area) falls off
  according to the inverse square law
• example
• Two light bulbs (AB) are equally as bright. Bulb
  B is placed 3 times further away. How does its
  brightness compare?
• Brightness ? 1/d2
• Brightness ? 1/32 1/9 as bright

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Color

• Additive primary colors for LIGHT
• (NOT like mixing paints---thats subtractive)
• Primary colors red, green, blue
• For light
• Red green yellow
• Red blue magenta
• Blue green cyan
• All colors white light

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Why does a green shirt appear green?
ROY G BIV

• White light on a green shirt will reflect green
  and absorb all the other colors
• What if a red light shines on a green shirt?
• There are no green wavelengths to reflect, so it
  absorbs the red and appears black

Red
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Colorreflection and absorption

• A color will appear a particular color because it
  reflects that wavelength of light (and absorbs
  all others)

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Colorsubtractive color mixing (like paints)

• More complex
• Think about which wavelengths can be reflected.
• M Y R
• C Y G
• C M B

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Polarizationtransmission of light such that
electric field waves are oriented in one direction
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Tyndall Effect

• Scattering of light through particles
• Higher frequency light scatters more (Why our
  sky appears blue)
• Lower frequency light scatters less (why sunsets
  appear red)

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Doppler Effect

• Motion of an object that emits or absorbs light
  causes a shift in the observed spectrum
• Receding objects spectrum red-shifts, so
  observed wavelength longer than normal
• Approaching objects spectrum blue-shifts, so
  observed wavelength is shorter than normal

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How Light is Emitted Black Body Radiation

• Ideal object that gives off radiation
• Perfectly absorbs all radiation, then re-emits
  radiation depending on temperature
• Hot object appears bluer, cold object appears
  redder

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Planck Curve Brightness of a black body
spectrum