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 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)

5. Interaction between Light and Matter in the Universe
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)
About 1s after the Big Bang, the Universe comprised a soup of protons, neutrons, electrons, and photons

6. Interaction between Light and Matter in the Universe
Where did the first 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 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

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 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? No.
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.

13. 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.

14. 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.
Note that, when describing Compton scattering, I refer to light as photons, that is as particles. Indeed, as we shall we, the Compton scattering effect established that light has particle-like properties.

15. 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? No.
Compton scattering demonstrates light has particle-like properties.
Note that, when describing Compton scattering, I refer to light as photons, that is as particles. Indeed, as we shall we, the Compton scattering effect established that light has particle-like properties.

16. 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?

17. 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 combine with protons to become H atoms. The Universe then became (largely) transparent.

18. 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 ± K?
What is a blackbody?
Why is the CMB a perfect blackbody?

19. 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

20. 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 ± K?
What is a blackbody?
Why is the CMB a perfect blackbody?

21. 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 ± K? The expansion of the Universe has Doppler shifted the CMB radiation.
What is a blackbody?
Why is the CMB a perfect blackbody?

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

23. Interaction between Light and Matter in the Universe
Where else does light in the Universe come from? Big Bang nuclear fusion in stars
2 protons – Deueterium – Heium nucleus (alpha particle)
Positron is the antimatter counterpart of the electron.

24. 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 ~1 million 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)

25. 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 ~1 million years to reach the surface and escape. Why? primarily electron scattering (throughout most of the solar interior) - absorption and re-emission by atoms (thin layer below surface)

26. Interactions between Light and Atoms
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?

27. Interactions between Light and Atoms
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? Yes.

28. Interactions between Light and Atoms
Spectral lines in Sunlight.

29. Interactions between Light and Atoms/Molecules
Light propagating from stars to the Earth can interact with gas and dust in the interstellar medium
Letter in Nature in 2004 reporting the discovery of N2 in the ISM
Hydrogen deuteride HD is diatomic molecule comprising 1 proton covalent bond with (1 proton + 1 neutron).
The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry1, 2, together with millimetre-wavelength observations3, 4 of interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet5 or infrared6 (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds1. Moreover, the N2 abundance does not explain the observed variations7 in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete8.

30. 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

31. 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

32. Interaction between Light and Matter in the Universe
Where else does light in the Universe come from? Big Bang nuclear fusion in stars
Vela supernova remnant

33. Interaction between Light and Matter in the Universe
Where else does light in the Universe come from? Big Bang nuclear fusion in stars exploding stars (supernova explosions)
Vela supernova remnant
Explosion ~12,000 years ago. Homo Sapiens emerged ~200,000 years ago

34. Interaction between Light and Matter in the Universe
Where else does light in the Universe come from? Big Bang nuclear fusion in stars exploding stars (supernova explosions)
Sirius A and B
Vela pulsar and pulsar wind nebula in X-rays
Sirius B was the 2nd discovered white dwarf, predicted in 1844 due to motion of Sirius A, detected in 1862

35. Interaction between Light and Matter in the Universe
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)
Sirius A and B
Vela pulsar and pulsar wind nebula in X-rays
Sirius B was the 2nd discovered white dwarf, predicted in 1844 due to motion of Sirius A, detected in 1862

36. 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.
Sirius B was the 2nd discovered white dwarf, predicted in 1844 due to motion of Sirius A, detected in 1862

37. 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.
Sirius B was the 2nd discovered white dwarf, predicted in 1844 due to motion of Sirius A, detected in 1862

38. 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.
Sirius B was the 2nd discovered white dwarf, predicted in 1844 due to motion of Sirius A, detected in 1862

39. 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

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

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

42. 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

43. 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

44. William Hyde Wollaston, 1766-1857
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,
A lens with one convex and one concave side is convex-concave or meniscus. It is this type of lens that is most commonly used in corrective lenses.

45. 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,
(Fe)
(Fe)
(Hα)
(O2)
(O2)
(Fe)
(Fe)
(Fe)

46. 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

47. 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,
Gustav Kirchhoff,

48. 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,
Gustav Kirchhoff,

49. 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.