The Interaction of Light and Matter

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. 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

4. Interaction between Light and Matter in the Universe • Where does light in the Universe come from?- Big Bang - nuclear fusion in stars - exploding stars (supernova explosions) - cooling stellar remnants (white dwarfs, neutron stars)

5. Interaction between Light and Matter in the Universe • Where does light in the Universe come from? - Big Bang- nuclear fusion in stars - exploding stars (supernova explosions) - cooling stellar remnants (white dwarfs, neutron stars)

6. Interaction between Light and Matter in the Universe • Where does light in the Universe come from? - Big Bang • How do we know there was a Big Bang?

7. Interaction between Light and Matter in the Universe • Where does 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

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

9. Interaction between Light and Matter in the Universe • 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?

10. Interaction between Light and Matter in the Universe • 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? -electron scattering

11. 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.

12. 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.

13. Interaction between Light and Matter in the Universe • 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?

14. Interaction between Light and Matter in the Universe • 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? - temperature decreased to ~3000 K, permitting electrons to combined with protons to become H atoms

15. Interaction between Light and Matter in the Universe • 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?

16. 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

17. Interaction between Light and Matter in the Universe • 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? The expansion of the Universe has Doppler shifted the CMB radiation.

18. Interaction between Light and Matter in the Universe • Where does light in the Universe come from? - Big Bang - nuclear fusion in stars - exploding stars (supernova explosions) - cooling stellar remnants (white dwarfs, neutron stars)

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

20. Interaction between Light and Matter in the Universe • 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 ~100,000 years to reach the surface and escape. Why? - electron scattering (throughout most of solar interior) - absorption and re-emission by atoms (thin layer below surface) Schematic of the Sun

21. Interactions between Light and Electrons • 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 ~100,000 years to reach the surface and escape. Why? - electron scattering (throughout most of solar interior)- absorption and re-emission by atoms (thin layer below surface) Schematic of the Sun

22. Interactions between Light and Atoms • 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 ~100,000 years to reach the surface and escape. Why? - electron scattering (throughout most of solar interior) - absorption and re-emission by atoms (thin layer below surface). Does this interaction produce spectral lines?

23. Interactions between Light and Atoms • Spectral lines in Sunlight.

24. Interactions between Light and Atoms/Molecules • Light propagating from stars to the Earth can interact with - gas and dust in the interstellar medium

25. Interactions between Light and Atoms/Molecules • 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

26. Interactions between Light and Atoms/Molecules • 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

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

28. Interaction between Light and Matter in the Universe • Where does light in the Universe come from? - Big Bang - nuclear fusion in stars - exploding stars (supernova explosions) Vela supernova remnant

29. Interaction between Light and Matter in the Universe • Where does light in the Universe come from? - Big Bang - nuclear fusion in stars - exploding stars (supernova explosions) - stellar remnants (white dwarfs, neutron stars) Sirius A and B Vela pulsar and pulsar wind nebula in X-rays

30. Interaction between Light and Matter in the Universe • 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.

31. 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

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

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

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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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.