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Two Conditions for an object to emit Blackbody Radiation:

Two Conditions for an object to emit Blackbody Radiation: It must be at a uniform temperature. (one average speed for its atoms) It must be opaque. (otherwise it would produce emission lines). Properties of Blackbody Spectra:

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Two Conditions for an object to emit Blackbody Radiation:

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  1. Two Conditions for an object to emit Blackbody Radiation: It must be at a uniform temperature. (one average speed for its atoms) It must be opaque. (otherwise it would produce emission lines)

  2. Properties of Blackbody Spectra: Hotter objects have their peak emission at shorter wavelengths (bluer colors). Hotter objects emit more energy at all wavelengths (are brighter).

  3. Properties of Electron Orbits around the Nucleus: Electrons are only allowed in specific orbits at specific distances from the nucleus. A specific energy change is required for the electron to move to higher or lower orbits. Electrons want to be as close to the nucleus as possible (in the ground state). The spacing of allowed orbits is different for every element.

  4. Relation between Electron orbits and Photons: Electrons can gain or lose energy - move in or out orbits - by emitting or absorbing photons. The energy of the photon (frequency, color) emitted or absorbed must equal the energy difference between orbits. Because the spacing of orbits is unique for every element, the set of colors emitted or absorbed is unique for every element.

  5. Comparison of Bound and Free Electrons: Bound electrons are transparent (they let most colors of light through). Free electrons are opaque (they can absorb any color of light).

  6. Properties of the Cosmic Microwave Background (CMB): It comes from every direction in the sky. It is incredibly distant (galaxy clusters in front distort it), and thus seen from a long time ago. It is uniform on the sky (the same in every direction). It is a perfect blackbody at a temperature of 2.7 Kelvin (comes from an opaque source).

  7. Conclusions About the CMB: • The CMB comes from a distant (ancient), constant temperature, opaque source of radiation, that was the same in every direction. • Therefore: A long time ago, the universe was a uniform temperature everywhere, and opaque to light. • Therefore: A long time ago, the universe consisted of a uniform, ionized plasma, at a temperature of a few thousand degrees.

  8. Since the time the CMB originated: • The universe has become less uniform (now different temperatures in different places), and has become transparent to light (we see stars and galaxies). • Therefore: The universe cooled from the time of the CMB until neutral atoms formed (the time of last scattering), after this the CMB was free to flow throughout the universe.

  9. Effects of Expansion on the Observed CMB: • The CMB is coming from incredibly far away (14 billion light years). • From Hubble’s Law, it will have an enormous redshift. • A redshifted blackbody spectrum looks just like a blackbody spectrum at a lower temperature. • The CMB is a blackbody spectrum at a temperature of a few thousand Kelvin, redshifted down to a few Kelvin by the expansion of the universe.

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