1. Radiation and Spectra Chapter 5
Light
Radiation and Spectra
Chapter 5

2. What is Light? Newton Maxwell Quantum Mechanics
Prism shows white light contains all colors
Light made of particles (photons)
Maxwell
Theory of electricity and magnetism
Light is electromagnetic waves
Produced by wiggling electrons
Radiation = production of light
Quantum Mechanics
Light is both: particle and wave

3. Waves Wavelength ( l ) Frequency ( f )
Distance between crests (or troughs)
Frequency ( f )
How often it repeats (wiggles up and down)
Measured in Hertz (Hz)
number of times per sec

4. Waves Speed c = 3 x 108 m/s Wavelength inversely related to frequency
c = lf
Wavelength inversely related to frequency
l = c / f
high frequency = short wavelength
low frequency = long wavelength

5. Particles as Waves
“Wave Packet”
particle/photon = localized wave

6. Properties of Light Color Carries energy (heat) Depends on frequency
blue = high frequency = short wavelength
red = low frequency = long wavelength
Carries energy (heat)
Photon energy
E = h f
high frequency = high energy = blue
low frequency = low energy = red
h = Planck’s constant

7. Red light has ____ than blue light.
A. larger frequency, energy, and wavelength
B. smaller frequency, energy, and wavelength
C. larger frequency and energy, but smaller wavelength
D. smaller frequency and energy, but larger wavelength

8. All types of light travel at the same speed -
Which of the following travels fastest? }
radio waves
infrared (heat) waves
microwaves
blue light waves
none of the above
All are
types of
light!
All types of light travel at the same speed -
the “speed of light”, c

9. Propagation of Light Photons travel in straight lines
energy spread over larger area at larger distances
produces 1/r2 decrease in brightness
Double distance - brightness decreases by 4

10. If a 100-watt light bulb is placed 10 feet away from
you, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter?
The closer one
The farther one
They will appear the same brightness
How much fainter will the far one appear compared to the close one?
Twice as faint
10 times fainter
100 times fainter
1000 times fainter
~ 1/r2

11. Electromagnetic Spectrum
Visible light:
red, orange, yellow, green, blue, indigo, violet (ROYGBIV)
Invisible Light:
Ultraviolet = bluer than blue
Infrared = redder than red
Other wavelengths:
Short: X-rays, gamma-rays
Long: microwave, radio

12. Which kind of electromagnetic radiation has a
wavelength longer than that of visible light?
A. infrared
B. ultraviolet
C. x-rays
D. gamma rays
E. none of the above

13. What’s the wavelength of my favorite radio station?
Link to:

14. Thermal Radiation All objects radiate (thermal radiation)
Objects made of atoms
Atoms (and their electrons) vibrate
Wiggling electrons radiate, producing light
Bigger objects produce more light
Higher temperature = stronger vibration
Hotter objects emit more light
Perfect absorber is black
Absorbed light (energy) heats object
Temperature increases until
emitted energy = absorbed energy
Emitted radiation called Blackbody Radiation
Thermal radiation emitted by most objects similar to blackbody

15. Blackbody Radiation Laws
Luminosity, L
L = energy emitted per second
Luminosity for a spherical object (a star)
L = 4p R2 s T4
R = radius (size) of star; T = temperature
double size, luminosity increases by 2x2 = 4
double temperature; luminosity increases by 2x2x2x2 = 16
Stefan-Boltzmann Law

16. Luminosity Temperature and Size Part I
WORKBOOK EXERCISE:
Luminosity Temperature and Size Part I
(pages in workbook)

17. Blackbody Radiation

18. Blackbody Radiation Laws
Color
Wavelength where most light emitted
lmax = 3 x 106 / T
T in Kelvin; lmax in nanometers (1 nm=10-9m)
Cool stars are red
Hot stars are blue
Color indicates temperature!
Wien’s Law
As T , Wavelength , Color = redder
As T , Wavelength , Color = bluer

19. Homework: WORKBOOK EXERCISE
Blackbody Radiation
(pages in workbook)

20. The graph above shows blackbody spectra for three different stars
The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature?
Star A
Star B
Star C
Because peak energy emission
occurs at shortest wavelength

21. Doppler Shift Originally discovered using sound waves Moving object
emits light with slightly different color
Frequency (pitch) of approaching object is higher
Blueshift
Wavelength shorter (shifted blueward)
Frequency (pitch) of receeding object is lower
Redshift
Wavelength longer (shifted redward)

22. Doppler Shift
Redshift
Blueshift

23. Spectroscopy Prism separates light into different colors
Continuous spectrum
contains all colors
Example: blackbody spectrum

24. Spectroscopy Absorption Line Spectrum
Some colors are missing (discrete lines)
Solar
Spectrum
N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

25. Spectroscopy Emission Line spectrum
Only certain colors are present (discrete lines)
Spectrum for each element unique (like fingerprints)

26. Model Atom Nucleus contains protons and neutrons
number of protons = element
(1 proton = hydrogen, 2 protons = helium, etc.)
number of neutrons about same as protons
Isotope = different number of neutrons
hydrogen
helium
Isotopes of hydrogen

27. Model Atom Electrons orbit nucleus
Number of electrons = number of protons
Ionization = removing electrons
Only certain orbits are allowed
hydrogen
helium

28. Atomic Absorption Atom absorbs photon energy
electron “jumps” to higher energy orbit
only certain discrete orbits are allowed
Atom can absorb only discrete colors (energies)

29. Atomic Emission Electron “jumps” to a lower energy orbit
Atom emits photon
can emit only discrete colors
same colors (wavelengths/energies) as absorption

30. Atomic Energy Levels Energy Levels Different for each element
each element has unique set of absorption/emission lines

31. Kirchoff’s Laws Continuous spectrum Emission line spectrum
Produced by hot solid (or dense gas)
Emission line spectrum
Produced by hot, low density gas
Absorption line spectrum
Produced when continuous source is viewed through cooler low density gas

32. Kirchoff’s Laws Absorption lines same wavelengths as emission lines
Gas can only absorb and emit at certain discrete frequencies/wavelengths/energies

33. WORKBOOK EXERCISE:
“Types of Spectra”
(pages in workbook)

34. If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see?
dark line absorption spectrum
bright line emission spectrum
continuous spectrum

35. Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight?
dark line absorption spectrum
bright line emission spectrum
continuous spectrum