Unit 2: Light & Spectroscopy

electromagnetic waves

• medium = electric + magentic field

• speed = 3e10⁸ m/s

• E = hf

waves

light is a type of wave

wave frequency

how often a crest washes over you

wave length

distance from one crest to the next

speed of light is constant inside….

a given medium

types of EM radiation

1. uv

2. visible

3. infrared

4. radio

visible light

• color of light determined by its wavelength

• white light is a mix of all colors

• separate individual colors with a prism

doppler effect

relative motion of source and observer shifts the wavelength

fraunhofer lines in sunlight discovered

dark lines appear in sunlight falling through a prism, spectrum of the sun

salt produces a…

yellow double line at 589 nm

kirchhoff’s laws

1. luminous solid or liquid emits light of all wavelengths: black body spectrum

2. light of a low density hot gas consists of bright emission lines: positive fingerprints

3. cool, thin gas absorbs wavelengths from a continous spectrum

   1. dark absorption (Fraunhofer) lines in continuous spectrum: negative fingerprints of its chemical elements, precisely at the same wavelengths as emission lines

kirchhoff’s laws: dark lines

• cool gas absorbs light at specific frequencies

• negative fingerprints of elements

kirchhoff’s laws: bright lines

• heated gas emits light at specific frequencies

• positive fingerprints of elements

spectral lines—fingerprints of elements

• identify elements on distant objects

• different elements yield different emission spectra

energy & power units

• energy has units Joule (J)

• rate of energy expended per unit time is called power, and has units Watt (W)

luminosity =

power radiated

stefan’s law

total energy radiated by a body at temperature T per second: P = A σ T⁴

stefan-boltzmann law

power radiated is proportional to the T (in K) to the 4th power

black body spectrum

objects emit radiation of all frequencies, but with different intensities

the higher the T of an object…

the higher its I_peak and f_peak

wien’s law

• peak of intensity curve will move with T

• T x wavelength = constant = 0.0029 K*m

  • higher the T, the smaller the wavelength, the higher the energy of the EM wave

measuring T

• find maximal intensity = wien’s law

• identify spectral lines of ionized elements = T

thermodynamics

describes systems with exceedingly many particles

spectroscopy

starlight tells us about T, velocity, chemical composition of stars

1st law of thermodynamics

energy is conserved

2nd law of thermodynamics

• entropy increases

• heat flows from hot to cold

consequence of thermodynamics

• universe will run out of things to do

  • heat death of universe

solar constant

amt of energy we receive on earth per second per area (W/m²)

sun’s blackbody spectrum

• peaks at 500 nm (green)

• used by Stefan to calculate T of sun

what EM waves can we see from the Earth’s surface

• visible light

• radio waves

• some IR light

vogel (1871)

1. sun is moving towards us--blue-shifted

2. accurate measurement of sun's rotation period

hot solids emit light at a range of…

frequencies

continuous spectrum

sunlight

chemical elements emit…

distinct lines

fraunhoffer lines

• dark lines appear in sunlight through a prism

• spectrum of sun