1. Light and The Electromagnetic Spectrum

2. 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.

3. 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”

4. 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

5. 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)

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

7. 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

8. Our sun at different wavelengths
Different parts of the sun will produce different wavelengths of electromagnetic radiation

9. 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

10. 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

11. 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

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

13. “Brightness”

14. 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

17. 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

18. 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

19. 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

20. As a graph, it looks like this…

21. (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)

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

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

24. 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

25. Spectral Line Features

26. Example: Spectral Lines of Hydrogen (Balmer Series)

27. 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’