Astro110-01 Lecture 14 Light and Matter 20/02/09 - - PowerPoint PPT Presentation

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Astro110-01 Lecture 14 Light and Matter 20/02/09 - - PowerPoint PPT Presentation

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Astro110-01 Lecture 14 Light and Matter 20/02/09 Habbal_Astro110-01 Lecture 14 1 What are the building blocks of matter? Or the parts of an atom Electron cloud Nucleus Atom proton neutron mass (proton) mass (neutron) 2000 x

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Astro110-01 Lecture 14 Light and Matter

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What are the building blocks of matter? Or the parts of an atom

Atom Electron cloud Nucleus proton neutron mass (proton) ≅ mass (neutron) ≅ 2000 x mass(e)

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Atomic Terminology

  • Atomic Number

– number of protons in nucleus

  • Atomic Mass Number

– number of protons + neutrons

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Atomic Terminology (cont’d)

  • Isotope:

– same # of protons but different # of neutrons (4He, 3He)

  • Molecules:
  • two or more atoms (H2O, CO2)
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How do light and matter interact?

  • Emission
  • Absorption
  • Transmission:

– Transparent objects transmit light. – Opaque objects block (absorb) light.

  • Reflection

– Scattering = random reflection

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Reflection and Scattering

Mirror reflects light in a particular direction Movie screen scatters light in all directions

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Interaction of Light with Matter

Interaction between light and matter determines the appearance of everything around us.

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Probing Matter through Light: The spectrum

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Different ways of visualizing a spectrum

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Three Types of Spectra: Kirchhof’s Laws

Continuous Emission Absorption

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Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum

Spectra of astrophysical objects are usually combinations of these three basic types

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Continuous Spectrum

  • The spectrum of a common (incandescent)

light bulb spans all visible wavelengths, without interruption.

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Emission Line Spectrum

  • A thin or low-density cloud of gas emits light only at

specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

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Absorption Line Spectrum

  • A cloud of gas between us and a light bulb can

absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

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How does light tell us what things are made of?

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Recall the parts of an atom

Atom Electron cloud Nucleus proton neutron

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Chemical Fingerprints

 Each type of atom has a unique spectral fingerprint

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Production of Emission Lines

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Chemical Fingerprints Energy Levels

Each type of atom has a unique set

  • f energy levels.

Each transition corresponds to a unique photon energy and frequency (or wavelength).

Energy levels of hydrogen E = h x f = h / λ

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Transitions yielding emission lines

  • Downward

transitions produce a unique pattern of emission lines.

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Transitions yielding absorption lines

  • Because those

atoms can absorb photons with those same energies, upward transitions produce a pattern

  • f absorption

lines at the same wavelengths.

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Production of Absorption Lines

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 Chemical fingerprints in a spectrum indicate which kinds of atoms are present

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Example: the Solar Spectrum

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How does light tell us the temperatures of planets and stars?

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Thermal Radiation

  • Nearly all large or dense objects emit thermal

radiation, including stars, planets, and you.

  • An object’s thermal radiation spectrum

depends on only one property: its temperature.

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Properties of Thermal Radiation

  • 1. Hotter objects emit more light at all frequencies

per unit area.

  • 2. Hotter objects emit photons with a higher

average energy.

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Wien’s Law

λ max = 2.9 106 / T (K)