What is light?
Electromagnetic energy of all colors
What can happen to light? terminology
Absorption
Emission
Transmission
Reflection
Refraction
The Wave Nature of Light
A photon is an electromagnetic wave
Wavelength vs. frequency
lambda x f = c
lambda in length f in
1/sec or hertz hz
Photon’s energy is
E = h x f h
is Planck’s constant
Wavelength regions : gamma-rays, x-rays, ultraviolet, visible, infrared, radio
The spectrum ranges from high energy gamma–rays to low energy radio waves
The interactions depends on the type of material.
Atomic structure: protons, neutrons, electrons
Phases: solid, liquid, gas, plasma
Hot thin gasses emit and cool thin gasses absorb
The pattern depends on the atomic structure example: nebula spectra
Continuous
Thermal or blackbody radiation
1. Hot solid, liquid or dense gas emit light at all wavelengths
2. Hotter objects emit
more total radiation per unit surface area
Stefan-Boltzman Law
Amount of energy per unit
surface E = sigma T4
sigma = 5.7 x 10-8
watt / (m2 K4)
3. Hotter objects emit
photons with a higher
average energy
Wien’s Law
lambdamax =
2,900,000 nm / T (K)
Measures radial velocity
(Observed lambda
– rest lambda)/ rest lambda
= radial velocity / speed of light
change in lambda/lambda0 = v/c
radial velocity of a star radial velocity curve
