Monday, October 1, 2012

Chapter 5: Light

5.1 Basic Properties of Light and Matter
  • Light is an electromagnetic wave. It is a form of energy composed of many different colors that combine to form white light.
  • Wavelength: the difference between adjacent crests. 
  • Amplitude: half the distance in height between a crest and a trough
  • Frequency: the number of crests that pass through a point each second
  • wavelength x frequency= the speed of light= constant
Particles of Light
  • Particles of light are called photons
  • Each photon has a wavelength and a frequency
  • The energy of a photon depends on its frequency
Wavelength, Frequency, and Energy
  • Lambda x f= c
  • Lambda= wavelength
  • F= frequency
  • c= 3.00 x 10^8 m/s = speed of light
  • E= h x f= photon energy
Matter
  • Atomic number= # of protons
  • Atomic mass number= # of protons + neutrons
  • Isotope= same number of protons, different number of neutrons
  • Molecule= compound of two or more atoms
How do light and matter interact?
  • Emission
  • Absorption
  • Transmission: Transparent objects transmit light, opaque objects absorb light
  • Reflection or scattering
Interactions of light and matter
  • Interactions between light and matter determine the appearance of everything around us.
5.2 Learning from Light
  • Spectra of astrophysical objects are usually combinations these three basic types: Emission line spectrum, Continuous Spectrum, and Absorption Line Spectrum
  • Continuous: The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.
  • Emission Line: 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.
  • Absorption Line: A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum. 
Chemical Fingerprints
  • Each type of atom has a unique set of energy levels
  • Each transition corresponds to a unique photon energy, frequency, and wavelength
  • Downward transitions produce a unique pattern of emission lines
  • Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths
  • Each type of atom has a unique spectral fingerprint
  • Observing the fingerprints in a spectrum tells us which kinds or atoms are present.
How does light tell us the temperature of planets and stars?
  • Nearly all large or dense objects emit thermal radiation, including stars, planets, and even you
  • An objects thermal radiation spectrum depends on one factor, its temperature
Properties of Thermal Radiation
  • Hotter objects emit more light at all frequencies per unit area
  • Hotter objects emit photons with a higher average energy
Doppler Effect
  • Generally measure the Doppler Effect from shifts in the wavelengths of spectral lines
  • Blue shift: objects moving towards
  • Red shift: Objects moving away
  • Effects tell us how fast an object is moving towards or away from us.

No comments: