The Interaction of Light and Matter. Learning Objectives Interaction between light and matter in...

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The Interaction of Light and Matter Slide 2 Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy: Motion from the Doppler effect Chemical composition (and more; e.g., density, temperature, and abundance) Magnetic Fields Discovery of spectral lines: Spectral lines in light from the Sun Empirical foundations of spectroscopy: Kirchoffs laws Slide 3 Where did the first light in the Universe come from? -Big Bang -nuclear fusion in stars -exploding stars (supernova explosions) -cooling stellar remnants (white dwarfs, neutron stars) Interaction between Light and Matter in the Universe Slide 4 Where did the first light in the Universe come from? -Big Bang How do we know there was a Big Bang? -Cosmic Microwave Background (CMB), revealing a time when the entire Universe was at a temperature of ~3000 K Interaction between Light and Matter in the Universe Slide 5 Why does the CMB map have an oval shape? Interaction between Light and Matter in the Universe Slide 6 The CMB comprises radiation from z = 1,089 (~380,000 years after the Big Bang), when the Universe first became transparent. Why was the Universe opaque for the first ~380,000 years after the Big Bang? Interaction between Light and Matter in the Universe Slide 7 Interactions between Light and Electrons Electron scattering occurs when a photon is scattered by a free electron: -in Thomson scattering, the scattering process is elastic; i.e., the electromagnetic wave does not lose any energy to the electron Does this interaction produce spectral lines? In Thomson scattering, the electron is made to oscillate by the electromagnetic field of the photon. The electron radiates most strongly in directions perpendicular to its oscillatory motion. Slide 8 Interactions between Light and Electrons An example of Thomson scattering is the reflection of electromagnetic radiation at long radio wavelengths by the Earths ionosphere. Slide 9 Interactions between Light and Electrons Electron scattering occurs when a photon is scattered by a free electron: -in Compton scattering, the process is inelastic; i.e., the photon loses a fraction of its energy to the electron Does this interaction produce spectral lines? Compton scattering demonstrates light has particle-like properties. Slide 10 The CMB comprises radiation from z = 1,089 (~380,000 years after the Big Bang), when the Universe first became transparent. Why did the Universe become transparent ~380,000 years after the Big Bang? Interaction between Light and Matter in the Universe Slide 11 The CMB is a perfect blackbody with a temperature of ~3000 K. Why, on the Earth, do we see a CMB blackbody temperature of 2.72548 0.00057 K? Interaction between Light and Matter in the Universe Slide 12 A brief review of Blackbody Emission A blackbody (hypothetical perfect absorber and emitter) has a specific intensity (units of energy per unit time per unit area per unit wavelength or frequency per unit solid angle; ergs s -1 cm -2 -1 sr -1 ): or Slide 13 Where else does light in the Universe come from? -Big Bang -nuclear fusion in stars Interaction between Light and Matter in the Universe Slide 14 Light-travel time from the center to the surface of the Sun is only 2.3 s. However, light produced at the center of the Sun takes ~1 million years to reach the surface and escape. Why? surface) Interactions between Light and Electrons Slide 15 At the photosphere of the Sun, the temperature is sufficiently low (~5777 K) that atoms can be neutral (e.g., H and He). Light propagating through this surface can be absorbed and reemitted by atoms through interactions with (bound) electrons. Does this interaction produce spectral lines? Interactions between Light and Atoms Slide 16 Spectral lines in Sunlight. Interactions between Light and Atoms Slide 17 Light propagating from stars to the Earth can interact with -gas and dust in the interstellar medium Interactions between Light and Atoms Slide 18 Light propagating from stars to the Earth can interact with -gas and dust in the interstellar medium -gas and dust in the interplanetary medium -gas and dust in the Earths atmosphere Interactions between Light and Atoms Slide 19 Light propagating from stars to the Earth can interact with -gas and dust in the interstellar medium -gas and dust in the interplanetary medium -gas and dust in the Earths atmosphere Interactions between Light and Atoms Slide 20 Where else does light in the Universe come from? -Big Bang -nuclear fusion in stars -exploding stars (supernova explosions) Interaction between Light and Matter in the Universe Vela supernova remnant Slide 21 Where else does light in the Universe come from? -Big Bang -nuclear fusion in stars -exploding stars (supernova explosions) -stellar remnants (white dwarfs, neutron stars) Interaction between Light and Matter in the Universe Vela pulsar and pulsar wind nebula in X-raysSirius A and B Slide 22 In summary, the interaction of light with matter can result in continuum and/or line (absorption or emission) radiation. It is because light interacts with matter that we can study matter in the Universe. Interaction between Light and Matter in the Universe Slide 23 Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy: Motion from the Doppler effect Chemical composition (and more; e.g., density, temperature, and abundance) Magnetic Fields Discovery of spectral lines: Spectral lines in light from the Sun Empirical foundations of spectroscopy: Kirchoffs laws Slide 24 Uses of Spectral Lines in Astronomy From spectral lines of light, we can deduce: -radial velocities or redshifts from the Doppler effect increasing Slide 25 Uses of Spectral Lines in Astronomy From spectral lines of light, we can deduce: -chemical compositions -effective temperatures of stars (Chap. 8) Slide 26 From spectral lines of light, we can deduce: -magnetic field strength from Zeeman splitting of spectral lines spatial dimension along slit slit Uses of Spectral Lines in Astronomy Slide 27 Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy: Motion from the Doppler effect Chemical composition (and more; e.g., density, temperature, and abundance) Magnetic Fields Discovery of spectral lines: Spectral lines in light from the Sun Empirical foundations of spectroscopy: Kirchoffs laws Slide 28 Discovery of Spectral Lines In 1802, the English chemist and physicist William Hyde Wollaston passed sunlight through a prism (like Newton and many others had done before him) and noticed for the first time a number of dark spectral lines superimposed on the continuous spectrum of the Sun. (Wollaston invented many optical devices, including the meniscus lens and the Wollaston prism. The latter separates light into two orthogonal linear polarizations.) William Hyde Wollaston, 1766- 1857 Slide 29 Identification of Spectral Lines By 1814, the German optician Joseph von Fraunhofer had cataloged 475 of these dark lines (today called Fraunhofer lines) in the solar spectrum. He labeled the strongest lines A to K, and weaker lines with lower-case letters. Fraunhofer determined that the wavelength of one prominent dark line in the Suns spectrum corresponds to the wavelength of yellow light emitted when salt is sprinkled in a flame. Thus was born the new science of spectroscopy. (Today, we know that this dark line is produced by the sodium atom, and is in fact a doublet but was spectrally unresolved at the time.) Joseph von Fraunhofer, 1787-1826 (O 2 )(H)(O 2 ) (Fe) Slide 30 Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy: Motion from the Doppler effect Chemical composition (and more; e.g., density, temperature, and abundance) Magnetic Fields Discovery of spectral lines: Spectral lines in light from the Sun Empirical foundations of spectroscopy: Kirchoffs laws Slide 31 Spectroscopy The foundations of spectroscopy were established by the German chemist Robert Bunsen and Prussian theoretical physicist Gustav Kirchhoff. They designed a spectroscope that passed the light of a flame spectrum through a prism to be analyzed. Bunsen designed the burner, which produced a hot and non-luminous flame. Burners that employ his basic design are still used today, and are know as Bunsen burners. Robert Bunsen, 1811-1899 Gustav Kirchhoff, 1824-1887 Slide 32 Spectroscopy The foundations of spectroscopy were established by the German chemist Robert Bunsen and Prussian theoretical physicist Gustav Kirchhoff. They designed a spectroscope that passed the light of a flame spectrum through a prism to be analyzed. Bunsen designed the burner, which produced a hot and non-luminous flame. Burners that employ his basic design are still used today, and are know as Bunsen burners. They found that the wavelengths of light emitted and absorbed by an element were the same. Kirchhoff determined that 70 dark lines in the solar spectrum correspond to 70 bight lines emitted by iron vapor. Robert Bunsen, 1811-1899 Gustav Kirchhoff, 1824-1887 Slide 33 Kirchhoffs Law Kirchhoff summarized the production of spectral lines in three laws, which are now known as Kirchoffs laws: Kirchhoffs laws are empirical laws. Our goal is to understand the physical processes behind these laws. The physical process behind Kirchoffs first law is the same as that responsible for blackbody radiation.