Dividing Light Into a Spectrum

Astronomers separate out light into its individual components using a diffraction grating or using a prism - then they analyze each part independently!

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81

blue 460 nm 81

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85

blue 460 nm 81green 530 nm 85

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83

blue 460 nm 81green 530 nm 85yellow 580 nm 83

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78

blue 460 nm 81green 530 nm 85yellow 580 nm 83orange 610 nm 78

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70

blue 460 nm 81green 530 nm 85yellow 580 nm 83orange 610 nm 78red 660 nm 70

The spectrum is continuous.

UV IR

Natural Spectra ????

Änuenue (rainbow)

Water dropletLight from the Sun

Shine Light through Hydrogen…

Shine Light through Hydrogen…

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Shine Light through Hydrogen…

E = hn = hc/l g l = hc/EE = hn = hc/l E = hn

h=Planck’s constant; n=frequency [Hz=1/s]; l=wavelength [m]

l (mm) = (mm=10-6m) E [eV]

1.24 [mm eV]

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E(3 g 2) = 12.07-10.19 = 1.89 eV

l(3 g 2) = ~ 0.656 mm1.891.24

l (mm) = (mm=10-6m) E [eV]

1.24 [mm eV]

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Shine Light through Hydrogen…

Thermal Radiation

Rules for Emission by Opaque Objects1. Hotter objects emit more total radiation per

unit surface area. Stephan-Boltzmann Law:

E = T4 ( = 5.7 x 10-8 [Watt/m2Kelvin4])

2. Hotter objects emit bluer photons (with a higher average energy.) Wien Law: l

max = 2.9 x 106 / T (K) [nm]

Rules for Emission by Opaque Objects1. Hotter objects emit more total radiation per

unit surface area. Stephan-Boltzmann Law:

E = T4 ( = 5.7 x 10-8 [Watt/m2Kelvin4])

2. Hotter objects emit bluer photons (with a higher average energy.) Wien Law: l

max = 2.9 x 106 / T (K) [nm]

Two kinds of Spectra: 1) Absorption• If light shines through a

gas, each element will absorb those photons whose colors match their electron energy levels.

• The resulting absorption line spectrum has all colors minus those that were absorbed.

• We can determine which elements are present in an object by identifying emission & absorption lines.

2) Emission Spectra• The atoms of each

element have their own distinctive set of electron energy levels.

• Each element emits its own pattern of colors, like fingerprints.

• If it is a hot gas, we see only these colors, called an emission line spectrum.

Lets look at continuous, absorption line, and emission line spectra –

Kirchhoff’s Laws

I. A hot, dense glowing object (solid or gas) emits a continuous spectrum.

Kirchhoff’s LawsII. A hot, low density gas emits

light of only certain wavelengths --

an emission line spectrum.

Kirchhoff’s Laws

III. When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum –

an absorption line spectrum.

Kirchhoff’s Laws

I III

II

Telescopes

Astronomical objects emit all of these different kinds of radiation in varying amounts

Mm/Submm

Maunakea’s height

Radio wavelength observations are possible

from Earth’s surface