Understanding EM Radiation

Electromagnetic Radiation
LACC §4.2, 4.3, 4.5

• Electromagnetic (EM) Radiation as a wave
• Electromagnetic (EM) Radiation as a particle
• Interactions between EM Radiation
(e.g. light) and Matter
All we know about objects beyond our solar
system comes (almost) solely from examining
the radiation (e.g. light) they emit.

Wednesday, February 17, 2010 1

is a wave
• EM radiation is pure energy (it has no mass)
• EM radiation results from the motion of
charged objects
• EM radiation travels at the speed of light
through a vacuum (and at lesser speeds
through matter)
• EM radiation is completely described by its
frequency, intensity, and direction of travel.


Waves: Diffraction

If the wavelength is of a If the wavelength does
similar size to a gap ... not match the size of
then the wave will the gap, then only a little
diffract as shown diffraction will occur
below. at the edge of the wave.

http://www.gcsescience.com/pwav37.htm


Waves: Interference

http://www.twow.net/ObjText/OtkCaLdQmB.htm


Waves: Interference

The diffraction pattern of light The same when the beam passes
observed on a distant screen through two identical closely
when a He-Ne beam passes spaced slits.
through a single narrow slit;

http://www1.union.edu/newmanj/lasers/Light%20as%20a%20Wave/
light_as_a_wave.htm


EM Radiation as a Wave
v = fλ
v = velocity
f = frequency
λ = wavelength

Q: What is v for light?
A: c, the speed of light
= 3x108 m/s
= 186,400 miles/s

http://www.bbemg.ulg.ac.be/UK/2Basis/freqlength.html


E.g. Light

http://www.uark.edu/ua/pirelli/html/color_freq_wavelength.html


is a particle

• atoms and molecules absorb and emit photons
• a photon is a single packet of EM energy


Atoms Emit Photons
E = hf

E = energy
h = Plank’s constant
f = frequency

h = 6.626x10-34 J•s
This makes Plank’s constant
the smallest(?) constant in
physics.

http://www.astrosociety.org/education/publications/tnl/35/light3.html


Atoms Absorb and Emit
Individual Photons

http://steve.ﬁles.wordpress.com/2006/03/Absorption%20emission.jpg


The EM Spectrum

•

Credit: Philip Ronan who has
given permission to copy,
distribute and/or modify this
document under the terms of
the GNU Free Documentation
License, Version 1.2 or any
later version.

•

Download site: Wikipedia:
Image:EM spectrum.svg.

http://www.nhn.ou.edu/~jeffery/course/c_energy/energyl/lec001.html


EM Radiation:
Wave or Particle?
Waves Particles
• interactions between • interactions between
waves results in particles result in
interference patters collisions
• radiate out from a • are “shot” out in
source speciﬁc directions
• can bend around • travel in straight
corners lines
• can bend around • are blocked by
obstacles obstacles


LACC §4.2, 4.3, 4.5
• Electromagnetic (EM) Radiation as a wave
(v = fλ)
• Electromagnetic (EM) Radiation as a particle
(i.e. photons, E=hf)
• Interactions between EM Radiation
(e.g. light) and Matter: absorption/emission
of EM radiation by atoms/molecules
system comes (almost) solely from examining
the radiation (e.g. light) they emit.


Spectroscopy
LACC §4.2, 4.3, 4.5

• Thermal Spectra: Wien’s Law, Stefan-
Boltaman Law
• Types of Spectra: there are 3 types of spectra
• Spectroscopy: what can it tell us?
system comes (almost) solely from examining the
electromagnetic radiation (e.g. light) they emit.


Thermal Radiation
http://astro.unl.edu/classaction/animations/light/meltednail.html
Blackbody Curves or Melting

http://cse.ssl.berkeley.edu/bmendez/
ay10/2002/notes/pics/bt2lf0612_a.jpg

http://astro.unl.edu/classaction/animations/light/bbexplorer.html
Blackbody Curves (NAAP)


Thermal Radiation

T = Temperature
λ = peak wavelength

Wein’s law

http://feps.as.arizona.edu/outreach/bbwein.html


Thermal Radiation

F= T 4
F = energy ﬂux
σ = Stefan-Boltzmann
constant
T = temperature

Stefan-Boltzmann
law
http://csep10.phys.utk.edu/astr162/lect/light/radiation.html


Types of Observed Spectra

http://instruct1.cit.cornell.edu/courses/astro101/lectures/images/lec07_04.jpg

http://astro.unl.edu/classaction/animations/light/threeviewsspectra.html
Three Views Spectrum Demonstrator


Atomic Energy Levels
of Hydrogen

http://www.daviddarling.info/
encyclopedia/H/
hydrogen_spectrum.html

http://astro.unl.edu/classaction/animations/light/hydrogenatom.htmlBlackbody Curves (NAAP)
Hydrogen Atom Simulator (NAAP)


EM Rad. & Space--Our Sun

http://www.weasner.com/etx/guests/2004/guests_spectra.html


EM Rad. & Space--Orion N.

http://mais-ccd-spectroscopy.com/Planetary%20Nebula.htm


EM Rad. & Space--M.W.


Images vs. Spectra

Which is better, the image of an astronomical
object, or the spectrum of an astronomical
object?

What about photometry?


Spectroscopy
LACC §4.2, 4.3, 4.5
• Types of Spectra: Continuous, Emission Line,
Absorption Line
• Thermal (or Blackbody) Spectra: Wien’s Law
(Temperature), Stefan-Boltaman Law (Power)
• Spectroscopy: Temperature, Composition,
Doppler Shift, Density
system comes (almost) solely from examining the
electromagnetic radiation (e.g. light) they emit.


LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch. 4, pp. 106-107: 23, 24.

• Ch 5: Tutorial Quizzes accessible from:
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
http://

ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of next class period.


Understanding EM Radiation

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