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Clicker Question:
The energy of a photon is proportional to its:
A: period
B: amplitude
C: frequency
D: wavelength
Clicker Question:
A star much colder than the sun would
A: red
B: yellow
C: blue
D: smaller
E: larger
The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4pR2, where R is the sphere's radius.
The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4
of the light
Each square gets 1/9
of the light
apparent brightness a 1
D is the distance between
source and observer.
The Doppler Effect
Applies to all kinds of waves, not just radiation.
at rest
velocity v1
velocity v2
velocity v1
velocity v1
velocity v3
you encounter
more wavecrests
per second =>
higher frequency!
fewer wavecrests
per second =>
lower frequency!
Doppler Effect
Demo: buzzer on a moving arm
Demo: The Doppler Ball
The frequency or wavelength of a wave depends on the
relative motion of the source and the observer.
Clicker Question:
If a star is moving rapidly towards Earth
then its spectrum will be:
A: the same as if it were at rest
B: shifted to the blue
C: shifted to the red
D: much brighter than if it were at rest
E: much fainter than if it were at rest
Example: Blackbody - the microwave
Spectroscopy and Atoms
How do you make a spectrum?
For light, separate white light into its colors using a glass prism or
"diffraction grating". For radiation in general, spread out the
radiation by wavelength (e.g car radio, satellite TV receiver).
How we know these things:
- Physical states of stars, gas clouds, e.g. temperature, density, pressure.
- Chemical make-up of stars, galaxies, gas clouds
- Ages of stars and galaxies
- Masses of stars, clouds, galaxies, extrasolar planets, rotation of galaxies,
expansion of universe, acceleration of universe.
All rely on taking and understanding spectra
Types of Spectra
1. "Continuous" spectrum - radiation
over a broad range of wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright at
specific wavelengths only.
3. Continuous spectrum with
"absorption lines": bright over a broad
range of wavelengths with a few dark
Kirchhoff's Laws
1. A hot, opaque solid, liquid
or dense gas produces a
continuous spectrum.
2. A transparent hot gas
produces an emission line
3. A transparent, cool gas
absorbs wavelengths from a
continuous spectrum,
producing an absorption line
The pattern of emission (or absorption) lines is a fingerprint of the
element in the gas (such as hydrogen, neon, etc.)
For a given element, emission and absorption lines occur at the same
Sodium emission and absorption spectra
Demo - Spectra
Demo - Spectrum of the sun
Spectrum of Helium (He) Gas
Discovered in 1868 by Pierre Jannsen during a solar eclipse
Subsequently seen and named by Norman Lockyer
Example: spectra - comet Hyakutake