Unlocking Light The key to understanding the Cosmos

Uses of Light  Astronomers use light as a tool to “dissect” celestial objects  Only direct information we can get!  Collect light with various instruments  Study light to determine sources

Light Sources  Atoms or parts of the atom are the source of all light  Behavior of particles in Atom generates light  What are these particles?

Parts of the Atom Proton Neutron Electron

Parts of the Atom  Atoms are building blocks of matter Nucleus  Protons – Positive Charge  Neutrons – Neutral Orbits  Electrons – Negative Charge

Building Blocks  Elements-pure substances  Atoms- smallest part of an element  Compound- chemically joined elements  Molecule- smallest part of a compound

Elements  Periodic Table  ID’s are Atomic Number (Z)  Z=number of protons  Atomic Mass (A) Number of Protons + Number of Neutrons

Elements  Neutral Atoms have same # of electrons and protons  Ions have lost or gained some electrons  Isotopes have lost or gained some neutrons

Get Excited  Electrons naturally orbit in lowest energy state  Ground State  If atom absorbs some energy the electron can move to a larger orbit  Excited Electron

Excited Atoms  Atoms gain energy  Excited (but not ionized) atoms  Electrons move away from nucleus but are still orbiting

Excited Atom Proton Neutron Electron

Excited Atoms  Electron absorbs energy to “jump up”  Electron releases energy to “fall down”  This Energy is Light!

Spectra  Light is released  This is the Spectra of the atom  Unique  Distinct Fingerprint for Elements and Compounds

Continuous Spectra  Rainbow  No Gaps in Colors  “White” light contains all colors  ROYGBIV

Emission Spectra  Dark Background  Few Bright Colored Lines  Spectral Lines  Unique for Each Element

Absorption Spectra  Similar to Continuous Spectra  Dark gaps where colors are missing  Also Spectral Lines  Unique for Each Element

Kirchhoff’s Rules  Describes conditions needed to form the 3 types of spectra  Can be related to Astrophysical conditions  We can infer information from type of spectra

Rule #1  Hot, opaque light source  Continuous Spectra  Light Bulb

Rule #2  Hot, transparent gas cloud  Emission Spectra  Neon Sign

Rule #3  Hot, opaque solid with a cool gas cloud between the source and viewer  Absorption Spectra  The Sun

Spectra Clues  Looking at the spectrum of a star can help us identify what elements are in it  Can also tell us about temperature and brightness  Gives clues to hidden processes within the star

Lots of Light  “Light” is any EM wave  EM= electromagnetic  Has both an electric part and a magnetic part  Not always “visible” to us  Entire spectrum is huge

Wave Anatomy Peak Trough Wavelength

 Length from one part of a wave to the next identical part  Can be measured in any length unit  Astronomers use Angstroms  1Å= meters ( m)

Frequency f  The # of waves that pass by a point in a given amount of time  # of waves per second  Measured in Hertz Hz

Wavelength and Frequency  Related!  Long Wavelength = Low frequency  Short Wavelength = High frequency  Wavelength increases, frequency decreases

Light Waves Wavelength x Frequency= Wave Speed  x f = c (speed of light)  c is same for every light wave  So what separates them is frequency and wavelength

Electromagnetic Spectrum  There is a continuous spectrum of light  Visible light (colors) are included in this  They are only a very very small part

EM Spectrum Gamma Rays X-rays Ultraviolet Visible Infrared Radio ← ← ← Wavelength increasing, Frequency decreasing

Energy of Light  Photons of light carry energy  This energy is related to wavelength and frequency  As frequency goes up, energy goes up, since more photons are arriving  As wavelength goes up, energy goes down

Energy of Light  If you double frequency, you double energy  Or, if you halve the wavelength, you double the energy

Summary  Light carries energy  Atoms make light  Light allows us to dissect objects that we can’t reach  Wavelength, frequency, and energy of light are related