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Lecture 4

ASTA02H3 Lecture Notes - Lecture 4: Black Body, Photon

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Diana Valencia

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ASTA02- Lecture 3 (Jan. 17th)
- Topics:
- What is a spectrum?
The distribution of photon energies coming from a light source
Spectra are seen by passing light through a spectrograph
We can tell how many photons are distributed based on wavelength
- Emission-Line Spectrum
A hot, low-density gas, in which the atoms are relatively isolated from each other will
emit an emission-line spectrum
ONLY emits light at particular wavelengths, giving the appearance of bright, discrete
emission lines
Each element has a unique set of emission-line spectrum (it can be seen as the
element’s ‘footprint’
Slightly different ELS for isotopes of the same element
- Absorption-Line Spectrum
Light from a continuous spectrum through a vessel containing a cooler gas shows a
continuous spectrum from the lamp crossed by dark absorption lines at wavelengths
- Kirchhoff’s Laws of Spectroscopy
A hot solid or hot, dense gas produces a continuous spectrum. Plenty of collisions
between the photons and the matter for the latter
- Blackbody Radiation
An object that absorbs all light (ALL wavelengths)
As it absorbs light, it heats up (characterized by temperature)
The perfect radiator
At higher temperature, more photons are emitted because it has higher energy (higher
temperature = higher energy)
At all wavelengths, a cooler star will have lower flux
The different flux patterns DO NOT cross in Blackbody Radiation
Color of the stars are correlated to their temperature. Hotter stars are closer to blue
because blueish colors have higher frequency and higher temperature = higher
frequency = higher energy
- Stefan-Boltzman Law
Energy emitted per second per area by a black body with Temperature (T)
Energy Flux = (Boltzman’s constant) x (T^4)
- Wein’s Law
o Relates peak wavelength and temperature: lambda peak = 2,900,000 nm.k/T
o Hotter objects are bluer
o Cooler objects are redder
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