PHY134 Introductory Astronomy Light and Matter 1

Summary Waves are a mathematical description of reaction of medium to local perturbation Exhibit interference and Doppler effect Energy conservation determines brightness of source at distance 2

Another Force Dominant force in most of physics: electromagnetism Coulomb force can be attractive or repulsive Opposite charges attract so most objects neutral Charge is conserved A charge creates and is affected by electric field 3

Magnets – and Light Moving charges create and are affected by magnetic fields (Ørsted 1820) Changing magnetic field creates electric field (Faraday 1831) Changing electric field creates magnetic field (Maxwell 1861) Leads to propagating waves with velocity Coincides with speed of light (Fizeau-Foucault 1850) Light is an electromagnetic wave! 4

Electromagnetic Spectrum Electromagnetic waves can have any wavelength What we see is limited by our eyes which are adapted to transparency of atmosphere What the Universe produces is not. Observing the Universe in many bands produces additional data 5

Heat Radiation A hot object radiates For dense dark objects radiation completely characterized by temperature – blackbody radiation Hotter objects are blue Wien 1893 Hotter objects radiate more. Stefan-Boltzmann 1879 flux at object 6

Example: Our Sun Measure Solar constant Compute luminosity Sun radius Luminosity is so Set to find Use Wien This is green 7

When Light Meets Matter Dense objects absorb light energy or reflect it. How much absorbed can depend on wavelength – dyes. Can learn composition from reflected spectrum Light scatters off tenuous matter (Rayleigh 1871) Scattering decreases with wavelength: blue scatters more than red 8

Scattering on Earth Atmosphere scatters blue light making sky glow blue and Sun appear yellow When we get more scattering – when Sun low in sky – lose green to scattering leaving Sun red 9

Scattering and Refraction Moon haloRainbow 10

Line Spectra Fraunhofer 1814: Sun’s spectrum has gaps Kirchoff-Bunsen 1859: Tenuous gas emits line spectrum Atoms and molecules emit/absorb at characteristic wavelengths when heated or ionized Line spectrum yields chemical composition At higher pressure and density lines broadened 11

Inside the Atom Rutherford 1909: Structure of the Atom is Keplerian Heavy nucleus of positive charge of size Orbited by light electrons of negative charge in orbits of size Atoms can bind by trading, sharing, or deforming their electrons. Chemistry is the science of electronic rearrangement Elements immutable because nucleus not affected 12