Light and The Electromagnetic Spectrum

Why do we have to study “light”?... Because almost everything in astronomy is known because of light (or some other form of electromagnetic wave) coming from stars, planets, galaxies, etc.

How We See the Universe We “see” the Universe in visible light Radiation in other forms is emitted too: –gamma rays –X-rays –Ultraviolet (UV) –Infrared (IR) –Microwaves and Radio All are forms of electromagnetic radiation What we know in astronomy is from all of these types of “light”

Electromagnetic Waves EM Waves are a response to changes in electrical and/or magnetic fields elsewhere. EM waves do NOT need a medium to travel through

Example EM Wave *Wavelength, (m or nm), is length from crest to crest *Frequency, f (Hz or s -1 ), is the number of wave crests per second that pass a given point * speed: v = f *ALL EM waves travel at the speed of light ( 3 x 10 8 m/s)

Example A wave has a frequency of 5 Hz and is traveling at 20 m/s. What is its wavelength? V = f 20 m/s = (5 Hz) (divide by 5Hz) = 4 m

Example A yellow light wave with a frequency of 5 x Hz. What is the wavelength of this yellow light? (all emag waves travel at speed of light) V = f 3x10 8 m/s = (5x10 14 Hz) (divide by 5x10 14 Hz) = 6 x m

Electromagnetic Spectrum Isaac Newton showed that ordinary sunlight could be split into many colors Each color corresponds to light of a specific wavelength (or frequency)

The Electromagnetic Spectrum High Energy*Low energy High Frequency*Low frequency Short wavelengths*Long wavelengths What we see (visible light) VIBGYOR microwaves

Our sun at different wavelengths classroom/multiwavelength_astronomy/mul tiwavelength_museum/sun.htmlhttp://coolcosmos.ipac.caltech.edu/cosmic_ classroom/multiwavelength_astronomy/mul tiwavelength_museum/sun.html Different parts of the sun will produce different wavelengths of electromagnetic radiation

ROY G BIV ROY G BIV (red, orange, yellow, green, blue, indigo, violet) Red light (next to infrared) is lowest energy visible light Violet light (next to ultraviolet) is highest energy visible light

“Brightness” Flux (rate of energy per area) falls off according to the inverse square law example: Two light bulbs (A&B) are equally as bright. Bulb “B” is placed 3 times further away. How does its brightness compare? Brightness  1/d 2 Brightness  1/3 2 = 1/9 as bright

Doppler Effect Motion of an object that emits or absorbs light causes a shift in the observed spectrum Receding objects: spectrum ‘red-shifts’, so observed wavelength longer than normal Approaching objects: spectrum ‘blue- shifts’, so observed wavelength is shorter than normal

Earth’s atmosphere is transparent to visible light and radio waves The atmosphere is opaque to other forms of radiation –Air ionized by X-rays and gamma-rays –UV absorbed by ozone –IR absorbed by carbon dioxide and water vapor Atmospheric “Windows”

How Light is Emitted: ‘Black Body’ Radiation Ideal object that gives off radiation Perfectly absorbs all radiation, then re-emits radiation depending on temperature Hot object appears ‘bluer’ cold object appears ‘redder’

Planck Curve: Brightness of a black body spectrum

Wavelength of spectrum’s peak found from Wien’s law T = 0.29 cm·K Integrated brightness emitted found from Stephan-Boltzmann law E =  T 4 (energy per area per time)

The Spectrum of the Sun Black body continuum What are these dark lines?

Fraunhofer lines in the Solar spectrum –absorption of specific wavelengths by cool gas in front of a black body radiator

Observed Spectra Absorption lines: occur when a cool gas lies in the line-of-sight between a hot object and the observer Emission lines: occur in hot gases (a cooling mechanism), best seen toward dark background

Atoms absorb and emit wavelengths of light specific to each chemical element This evidence is the basis for formation of quantum theory Electrons in atoms absorb or emit photons of light of a particular wavelength, and change their orbital energy level Atomic Radiation and Absorption: Spectral Lines

The Bohr Model for Hydrogen Hydrogen’s single electron orbits the nucleus of the atom in ‘quantized’ levels (lowest energy level is the ground state) Electron that moves from high level to low level emits a photon of a specific energy Electron that absorbs a photon of a specific energy is allowed to move from low to high energy level

Spectral Line Features

Example: Spectral Lines of Hydrogen (Balmer Series)

Hydrogen Visible lines are known as Balmer series, involving transitions to and from the n=2 level Transitions to and from the n=1 level are Lyman series, and are primarily in UV If energy of photon is high enough, the electron can escape the atom, causing it to be ‘ionized’