Electromagnetic Radiation

1
Electromagnetic Radiation

• Topics
• All about light
• Kirchhoffs and Wiens laws
• Doppler effect
• Demo.

Motivation Learn about light. Learn about light
as a tool.
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What is Light?
Light is a form of energy transport. A beam of
light can be decomposed to individual components
called photons. All light, no matter how much
energy per photon, travels at the same
speed. c3108 m/s An individual photon can be
described as having a frequency (?) and a
wavelength (?). These are related to each other
by c??
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Electromagnetic Spectrum

• Visible light
• A prism produces a spectrum
• Visible colors (red, orange, yellow, green, blue,
  and violet) in roughly equal proportions gives
  white light.
• Black is the lack of color.
• Visible photons range in ? from 400 nm (0.004mm)
  to700 nm (0.007 mm).
• Electromagnetic Spectrum
• Radio
• Microwave
• Infrared
• Visible
• Ultraviolet
• X-Ray
• Gamma Ray

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Light and Matter
Emission An object creates and emits light
(e.g., light bulb filament). Absorption When an
object absorbs light, transmitting the energy to
the internal energy of the object. Transmission
Light simply passes through an
object. Reflection/scattering Light bouncing off
matter in the same general direction is
reflection when the bouncing is in random
directions it is called scattering.
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Kirchhoffs Laws

• Kirchhoffs laws summarize how the three types of
  spectra are produced. Without explaining the
  underlying physics
• Continuum spectrum
• A hot, dense glowing object (a solid or dense
  gas) emits a continuous spectrum.
• Emission spectrum
• A hot, low-density gas emits light of only
  certain wavelengths - a bright line spectrum.
• Absorption spectrum
• When light having a continuous spectrum passes
  through a cool gas, dark lines appear in the
  continuous spectrum.

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Wiens Law

• An intensity/wavelength graph, a thermal
  spectrum, of an object emitting electromagnetic
  radiation can be used to determine its
  temperature.
• Therefore, the color of a star tells us about its
  surface temperature.
• A quantitative derivation is given by Wiens Law
  
• lmax 2,900,000/T or T 2,900,000/lmax
• where T is the temperature in Kelvin and lm is
  the wavelength where the thermal spectrum peaks
  in intensity in nanometers (nm).

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Power Output

• How much thermal energy is being emitted (per
  square meter) from an object with at temperature
  T?
• E/m2 sT4
• Where s Stefan-Boltzmann constant.
• While energy emitted by a sphere is
• E4pR2 sT4

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The Doppler Effect More Wave Behavior
?o is the wavelength emitted by the object. ? is
the wavelength we observe. vr is the radial speed
of the emitting object. c is the speed of light,
300,000 km/s.
vt
vr
v
To Earth
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Roll-up on waves!!!!

• What we can learn from light.
• General conditions (Kirchhoffs Laws)
• Temperature (Wiens Law)
• Speed-overall (Doppler)
• Speed-rotation (Doppler)
• Speed-surface activity (Doppler)
• Composition (spectral lines)
• Density (excitations)
• Magnetic fields (spectral lines splitting)
• more