# Light. What is Light? The third form of energy The only thing astronomers study Electromagnetic radiation The thing that our eyes detect How radio works.

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Light

What is Light? The third form of energy The only thing astronomers study Electromagnetic radiation The thing that our eyes detect How radio works

Electric Fields Force Fields! Just like on Star Trek! Sort of… 2 21 r qq kF 

Magnetic Fields Accelerating Electrons create Magnetic Fields!

EM Radiation! Changing E fields create B fields

Waves Waves carry energy

Waves

ABCD Longer wavelength means: A. lower frequency and lower energy B. higher frequency and lower energy C. lower frequency and higher energy D. Higher frequency and higher energy

The Spectrum The visible part of the spectrum is rather small

Particles of Light The photoelectric effect

InteractionsReflectionEmission Absorption

Thermal Emission Everything with a temperature emits light Power per unit Area (Flux) and color depend only on the temperature

BlackbodyKnmT  6 max 109.2 4 TF 

Blackbody AB C

Question Objects A and B emit like blackbodies. The color at the peak is the same for both objects. A is emitting twice as many total photons as B. A. A is hotter that B B. A is colder than B C. A and B are at the same temperature D. A is smaller than B

Atomic Structure Classical View Electrons are in orbits about the nucleus

Emission Lines Red Low Energy Blue High Energy Every substance has its own spectral fingerprint

Reflection and Scattering

Absorption A perfect absorber is a black body Absorbs all but red

Absorption

Absorption Atoms and molecules can absorb photons

ABCD In the Sun, the 4 to 2 hydrogen transition produces photons with λ=486.1. If the Sun were twice the temperature, λ would be: A. half as big B. 4 times as big C. the same D. twice as big

QUIZ!

Why is the Sky Blue? For the same reason that sunsets are red.

Putting it All Together Spectroscopy!

Doppler shift

Measuring Velocity If we know the REST wavelength of an emission line…

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