The Interaction of Light and Matter

Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy:

Motion from the Doppler effectChemical 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

Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy:

Motion from the Doppler effectChemical 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

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)

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)

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?

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

Interaction between Light and Matter in the Universe

Why does the CMB map have an oval shape?

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?

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

Interaction between Light and Matter in the Universe

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.

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.

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

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

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?

Interaction between Light and Matter in the Universe

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

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.

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)

Interaction between Light and Matter in the Universe

Where does light in the Universe come from?- Big Bang- nuclear fusion in stars

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

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

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). Does this

interaction produce spectral lines?

Interactions between Light and Atoms

Spectral lines in Sunlight.

Interactions between Light and Atoms

Light propagating from stars to the Earth can interact with - gas and dust in the interstellar medium

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

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

Interactions between Light and Atoms/Molecules

Where does light in the Universe come from?- Big Bang- nuclear fusion in stars

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)

Interaction between Light and Matter in the Universe

Vela supernova remnant

Where 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

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

Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy:

Motion from the Doppler effectChemical 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

Uses of Spectral Lines in Astronomy From spectral lines of light, we can deduce: -

radial velocities or redshifts from the Doppler effect

increasing λ

Uses of Spectral Lines in Astronomy From spectral lines of light, we can deduce: -

chemical compositions- effective temperatures of stars (Chap. 8)

From spectral lines of light, we can deduce: -magnetic field strength from Zeeman splitting of spectral lines

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Uses of Spectral Lines in Astronomy

Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy:

Motion from the Doppler effectChemical 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

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

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

Learning Objectives Interaction between light and matter in the Universe. Some uses of spectral lines in astronomy:

Motion from the Doppler effectChemical 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

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

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

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.