Wave property of light

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Wave property of light
Waves can carry energy Wavelength (l) distance
between successive crests (or troughs) Frequency
(f) of waves passing a point in 1 second Wave
speed (v) wavelength x frequency v l x
f vsound 340 meters/second vlight 3x108
meters/second
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More Newton
Showed that light is composed of many
colors Different l means different color (and
vice versa) Visible light has short
wavelengths Blue l 4 x 107 meters (400
nm) Green l 5.5 x 107 meters (550 nm) Red l
7 x 107 meters (700 nm) 1 nanometer 10-9
meters Visible light range 400 to 700 nm
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Electromagnetic Spectrum

• Visible 400-700 nm
• Usable 0.001 nm 10 km!
• Atmospheric Windows
• Earths atmosphere absorbs and reflects radiation
  at several wavelengths
• From the ground, we only detect visible and radio
• For other wavelengths, we must observe in a place
  above most or all of the atmosphere
• Mountains Near IR
• Planes Far IR
• Balloons UV, X-ray
• Space everything including gamma ray

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Measuring Light

• Luminosity
• Total rate of energy emission
• Intrinsic to the star
• L energy/time ergs per second
• Brightness
• Depends on distance from object
• Rate of energy passage through a fixed area
• B (ergs/second) / of cm2 covered L/4pR2
• B L/R2 Inverse Square Law of Light

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The particle nature of light
Late 1800s Early 1900s Quantum
Mechanics Light can be thought of as being made
up of particles called PHOTONS The energy of a
photon is proportional to frequency Ephoton
f High frequency high energy photon Low
frequency low energy photon Bright light
lots of photons
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Continuous Spectrum

• Emitted by hot bodies (also called thermal
  radiation or black body radiation)
• A blackbody is a perfect radiator
• Emits photons at all energies (a continuous
  spectrum)
• Shape of the spectrum is solely determined by the
  objects temperature

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Properties of thermal radiation
Increase temperature particles move
faster interact at higher energies more of the
higher energy photons produced Wiens
Law Hotter objects emit higher energy (bluer)
photons lmax 3x106 nm / T (T in Kelvin)
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Properties of thermal radiation
Hotter objects emit more photons, so hotter
objects are brighter objects Energy emitted per
unit surface area T4 Double an objects
temperature, and it emits 16 times as much
energy! (16 24) Triple the temperature, and it
emits 81 times as much energy!! (81
34) Luminosity of an object depends on?
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Properties of thermal radiation
Stefan-Boltzmann Law Luminosity depends on
temperature and surface area L R2T4