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

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 microwaves What we see (visible light) VIBGYOR

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

Our sun at different wavelengths http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_museum/sun.html Different parts of the sun will produce different wavelengths of electromagnetic radiation

Example EM Wave Wavelength, , is length from crest to crest Frequency, f is the number of wave crests per second that pass a given point Speed formula: v = f  ALL electromagnetic waves travel at the speed of light = 3 x 108 m/s

Example 20 m/s = (5 Hz)  (divide by 5Hz)  = 4 m 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 3x108 m/s = (5x1014 Hz)  (divide both sides by 5x1014 Hz) A yellow light wave with a frequency of 5 x 1014 Hz. What is the wavelength of this yellow light? (Remember: all emag. waves travel at speed of light) v = f  3x108 m/s = (5x1014 Hz)  (divide both sides by 5x1014 Hz)  = 6 x 10-7 m

Planck Curve: Brightness of a black body spectrum Frequency and energy are directly related. Hot object appears ‘bluer’, Cooler objects appear ‘redder’

“Brightness”  

Atmospheric “Windows” 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

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

Spectroscopy and Spectral Lines 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

How is light produced? The Bohr Model Excited electrons in higher energy levels will fall to low level and will emit a light wave of a specific energy and color Different transitions are different colors Different wavelengths are different elements (it’s unique to that element….like a fingerprint) The Bohr Model

As a graph, it looks like this…

(energy per area per time) Wavelength of spectrum’s peak found from Wien’s law T = 0.29 cm·K Integrated brightness emitted found from Stephan-Boltzmann law E = T4 (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

Atomic Radiation and Absorption: Spectral Lines 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

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’