Chapter 5 Notes Jessica Brandon

Chapter 5: Light: The Cosmic Messenger

***5.1 Basic Properties of Light and Matter***

·      Light is an electromagnetic wave

·      Light is also a particle

Photons: “pieces” of light, each with precise wavelength, frequency, and energy.

Atomic Terminology

·      Atomic Number = # of protons in nucleus

·      Atomic Mass Number = # of protons + neutrons

·      Isotope: same # of protons but different # of neutrons. (⁴He, ³He)

***How do light and matter interact?***

·      Emission

·      Absorption

·      Transmission

·      Reflection or Scattering

·      Terminology

·      Transparent: transmits light

·       Opaque: blocks (absorbs) light

5.2 Learning from Light

***How does light tell us what things are made of?***

·      Electrons in atoms have distinct energy levels.

·       Each chemical element, ion, molecule, has a unique set of energy levels.

Distinct energy levels lead to distinct emission or absorption lines.

Chemical Fingerprints

·      Every atom, ion, and molecule has a unique spectral “fingerprint”

·      We can identify the chemicals in gas by their fingerprints in the spectrum.

·      With additional physics, we can figure out abundances of the chemicals, and much more.

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

Thermal Radiation

·      Nearly all large or dense objects emit thermal radiation, including stars, planets, you…

·      An object’s thermal radiation spectrum depends on only one property: its temperature

Two 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.

***How does light tell us the speed of a distant object? ***

The Doppler Effect: Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:

·      Doppler Effect Summary

·      Motion toward or away from an observer causes a shift in the observed wavelength of light:

·      blueshift (shorter wavelength) Þ motion toward you

·      redshift (longer wavelength) Þ motion toward you

·      greater shift Þ greater speed

5.3 Collecting Light with Telescopes

***How do telescopes help us learn about the universe? ***

·      Telescopes collect more light than our eyes Þ light-collecting area

·      Telescopes can see more detail than our eyes Þ angular resolution

·      Telescopes/instruments can detect light that is invisible to our eyes (e.g., infrared, ultraviolet)

Angular Resolution

•The minimum angular separation that the telescope can distinguish.

***Why do we put telescopes into space? ***

·      Recall our 1-to-10 billion scale:

·      Sun size of grapefruit

·      Earth size of ball point, 15 m from Sun

·      Nearest stars 4,000 km away

·      Hubble orbit microscopically
  above ball- point size Earth

Telescopes in space solve all 3 problems.

·      Location/technology can help overcome light pollution and turbulence.

·      Nothing short of going to space can solve problem of atmospheric absorption of light.

***How is technology revolutionizing astronomy? ***

·      Adaptive optics: Rapid changes in mirror shape compensate for atmospheric turbulence.

·      Interferometry: Allows two or more small telescopes to work together to obtain the angular resolution of a larger telescope.