1. The Interaction of Light and Matter

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: Kirchoff’s laws

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

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

5.  Why does the CMB map have an oval shape? Interaction between Light and Matter in the Universe

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

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.

8. Interactions between Light and Electrons  An example of Thomson scattering is the “reflection” of electromagnetic radiation at long radio wavelengths by the Earth’s ionosphere.

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.

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

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

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 Å -1 sr -1 ): or

13.  Where else does light in the Universe come from? -Big Bang -nuclear fusion in stars Interaction between Light and Matter in the Universe

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

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

16.  Spectral lines in Sunlight. Interactions between Light and Atoms

17.  Light propagating from stars to the Earth can interact with -gas and dust in the interstellar medium Interactions between Light and Atoms

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 Earth’s atmosphere Interactions between Light and Atoms

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 Earth’s atmosphere Interactions between Light and Atoms

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

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

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

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: Kirchoff’s laws

24. Uses of Spectral Lines in Astronomy  From spectral lines of light, we can deduce: -radial velocities or redshifts from the Doppler effect increasing λ

25. Uses of Spectral Lines in Astronomy  From spectral lines of light, we can deduce: -chemical compositions -effective temperatures of stars (Chap. 8)

26.  From spectral lines of light, we can deduce: -magnetic field strength from Zeeman splitting of spectral lines λ spatial dimension along slit slit Uses of Spectral Lines in Astronomy

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: Kirchoff’s laws

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

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 Sun’s 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 )(Hα)(O 2 ) (Fe)

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: Kirchoff’s laws

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

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

33. Kirchhoff’s Law  Kirchhoff summarized the production of spectral lines in three laws, which are now known as Kirchoff’s laws:  Kirchhoff’s laws are empirical laws. Our goal is to understand the physical processes behind these laws. The physical process behind Kirchoff’s first law is the same as that responsible for blackbody radiation.