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PHY134Introductory Astronomy

Light and Matter

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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

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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

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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!

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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

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Heat Radiation• A hot object radiates• For dense dark objects

radiation completely characterized by temperature – blackbody radiation

• Hotter objects are blueWien 1893

• Hotter objects radiate more. Stefan-Boltzmann 1879

flux at object

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

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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

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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

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Scattering and RefractionMoon halo Rainbow

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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

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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