The Electromagnetic Spectrum and Observing for Educators

Transcription

1 YAAYS course: The Electromagnetic Spectrum and Observing for Educators February 19, 2008 Rich Kron

2 from last week: visible light, radio, infrared, X-rays etc. are all electromagnetic radiation, differing only by energy electromagnetic radiation can be considered to be a wave (with a certain wavelength), or a particle (photon) energy can be measured by frequency or by wavelength

3 almost everything we know about the Universe is decoded from the light we receive from distant objects Earth s atmosphere transmits at some wavelengths and absorbs at others a spectrum ( rainbow ) is a representation of the number of photons something emits according to the energy of each photon there are different types of spectra

4 Types of Spectra emission continuous absorption Discovering the Universe Chapter 4

5 Types of Spectra alternate titles: How Light is Created and Absorbed Properties of Glowing Objects by measuring spectra, we can tell a lot about how something radiates (how it works )

6 Bells and violin strings are examples of systems that oscillate at some particular frequency (also overtones, harmonics, modes). You can add energy to these systems if the energy is tuned to that frequency. You can also get energy out (e.g. moving air).

7 The Tacoma Narrows Bridge is another example of a system with a particular frequency of oscillation - energy was supplied by wind (1940). Bridges are now designed so they don t do this. Rather, the energy goes into numerous small vibrations that eventually turn into heat.

8 Lick Observatory, Mt. Hamilton, CA note the large fork of the telescope mounting - can it vibrate like a tuning fork?

9 Like a violin string, atoms can absorb energy (photons) only at particular frequencies, and can emit energy (photons) only at particular frequencies. The emission or absorption of light corresponds to the transition of the atom between one state or energy level and another.

10 The spectrum of hydrogen shows energy only at specific wavelengths ( lines ). Ephoton = Eatom(initial state) - Eatom(final state) The structure of the spectrum of an atom - hydrogen, helium, argon, etc. - is related to the structure of the atom itself.

11 Niels Bohr Danish physicist Johann Balmer Swiss math teacher hydrogen spectrum: λ = const. n 2 / (n 2-4) Carlsberg brewery, Copenhagen

12 a low-density gas with energy put into it (for example, via electrical spark) emits light with a line spectrum. Gustav Kirchhoff, Robert Bunsen

13 Examples of line emission neon light street light lightning hot clouds of interstellar gas upper atmosphere of the Sun low-density gas near a black hole

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15 simulated spectrum of lightning on Jupiter

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17 spectrum of hot interstellar gas: emission lines

18 spectrum of hot dense gas near black hole: emission lines

19 all dense objects that are hotter than their surroundings radiate the spectrum of the radiation is continuous (no features )

20 Examples of continuous emission light bulb filament hot lava people high-density gas near a black hole interstellar dust particles the Universe

21 gas spirals in towards black hole, heats up to high temperature, radiates X-rays, UV, visible light

22 infrared image of interstellar dust particles in Cygnus

23 all-sky map - the Universe as revealed by a telescope tuned to microwaves

24 Why does low-density hot gas emit a line spectrum, while high-density hot material emits a continuous spectrum?

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26 the third kind of spectrum is called an absorption spectrum: light is observed at all wavelengths except specific ones lower layers in the Sun are denser and hotter, upper layers are less dense and transparent light from the lower layers (continuous spectrum) is absorbed by the cooler gas through which it passes

27 Spectrum of daylight = spectrum of Sun

28 spectrum of Sun-like star

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30 transparency of Earth s atmosphere visible infrared

31 A is the naked core of the evolving star that created the Ring Nebula

32 Summary: Types of Spectra hot, low-density gas emits a line spectrum lots of diagnostics: atom, ion, isotope; pressure; temperature; etc. hot, dense material (solid, liquid, dense gas) emits a continuous spectrum for a perfect radiator (black body), get only temperature astronomical sources are very often one of these types