1. ASTR 1040 – September 21 Second Homework Due next Thursday 28th
Planetarium Next Tuesday September 26
First Exam October 5
Observatory Opportunity Next Tuesday 26th
Website

2. Nature of Light Light is a flux of particles called photons
Each photon is both a particle and a wave (a packet of waves)
250 years after Newton we still don’t understand it
Electromagnetic Theory (Maxwell’s Equations) 1860’s
Quantum Electrodynamics 1948 Feynman
Each photon has:
direction
wavelength
polarization

3. Light Waves l lambda is lower case Greek L stands for length
Each photon is a sine wave moving at the speed of light
Wavelength is usually measure in Angstroms
1Å = 10-8cm =10-10m
about the diameter of an atom.
And 10Å = 1nm
Electric and Magnetic
Fields Sloshing Back
And Forth

4. Color RED 7000Å YELLOW 5500Å VIOLET 4000Å
Wavelength Determines Color of Light
Color is the eye’s response to different wavelengths
Color is a physiological effect
A photon can have any wavelength
RED 7000Å
YELLOW 5500Å
VIOLET 4000Å

5. Electromagnetic Spectrum
visible is tiny chunk of em spectrum

6. Parts of EM Spectrum Radio l > 1mm (107A)
Infrared 1mm> l > 10000A
Visible 10,000A > l > 3500A
Ultraviolet 3500A > l > 100A
X-ray 100A > l > 0.1A
Gamma-ray 0.1A > l

7. Question
What range of wavelength can the average human eye see and what color is each side of the spectrum? A) 400nm-800nm, redder to bluer B) 500nm-700nm, bluer to redder C) 400nm-700nm, bluer to redder D) 300nm-600nm, redder to bluer E) None of the above

8. Answer
What range of wavelength can the average human eye see and what color is each side of the spectrum? A) 400nm-800nm, redder to bluer B) 500nm-700nm, bluer to redder C) 400nm-700nm, bluer to redder D) 300nm-600nm, redder to bluer E) None of the above Answer: C

9. Speed of Light Speed of Light c = 3x108m/s That’s a very odd statement
2 cars at 65mph
1 car at 130mph
Cover same distance in same amount of time
The Relative speeds are the same

10. Relativity NO!!! .8c .8c Clearly Approaching each other at 1.6c
v always less than c
if velocities << c, then v=v1+v2
per Einstein
(Concept of time and space changes)

11. Frequency l l l l Moves l during each cycle
Frequency is the number of cycles per second, n
Greek “nu”
Moves distance l for each of n cycles each second

12. Frequency (2)
300MHz = 1m wavelength
Yellow Light = 600 trillion Hertz

13. Question An x-ray has a wavelength of 100Å
(10nm, 1x10-8m). What is it's frequency, in cycles per second? (aka Hertz)
A. 3x1016
B. 1.5x1016
C. 3x1013
D. 1.5x1013

14. Answer
An x-ray has a wavelength of 100Å (10nm, 1x10-8m). What is it's frequency, in cycles per second? (aka Hertz)
A. 3x1016
B. 1.5x1016
C. 3x1013
D. 1.5x1013
Answer: A. (3E8m/s)/(1E-8m)=3E16 Hz

15. Energy of a Photon h = 6.63x10-34 J s Planck’s Constant
energy of yellow photon
Sunlight is 104 W/m2
Outside we have 1023 photons/m2/s hit us

16. Question
How many times more energy is there in an x-ray photon at 100A than the infrared light photons emitted by every living human? (Assuming 10,000nm wavelength of infrared light).
A. Ten times as powerful.
B. A hundred times more powerful.
C. A thousand times more powerful.
D. 1x1012 (a trillion) times more powerful.
E. 1x1015 (a quadrillion) times more powerful.

17. Answer
How many times more energy is there in an x-ray photon at 100A than the infrared light photons emitted by every living human? (Assuming 10,000nm wavelength of infrared light).
A. Ten times as powerful.
B. A hundred times more powerful.
C. A thousand times more powerful.
D. 1x1012 (a trillion) times more powerful.
E. 1x1015 (a quadrillion) times more powerful.
Answer: C. 10,000nm/10nm = 1000

18. Spectroscopy
Spectrum is plot of number of photons as a function of wavelength
Tells us huge amounts about nature of object emitting light.

19. Thermal Radiation Planck’s Law
Temperature Determines Where Spectrum Peaks
Position of Peak Determines Color

20. Blue is Hotter than Red Optically Thick, But hot
Sun almost “white hot”
Burner “red hot”
Desk “black hot”
Ice Cube “black hot”

21. Question
A star with a temperature of 100,000K has what color to the naked eye?
White
Yellow
Orange
Red

22. Wien’s Law Å As T rises, l drops Bluer with temperature (T in Kelvin)
300K 100,000A Earth
Sun
X-ray source

23. Question
How many times smaller would the peak wavelength be for a star twice as hot as the Sun? (Remember the sun is 5500K)
A. Twice as long
B. Half as long
C. Four times as long
D. A fourth as long

24. Answer
How many times smaller would the peak wavelength be for a star twice as hot as the Sun? (Remember the sun is 5500K)
A. Twice as long
B. Half as long
C. Four times as long
D. A fourth as long
Answer: B. Since peak wavelength is a function of the inverse of temperature, doubling the temp of a star would cause it's peak wavelength to cut in half.

25. Stefan-Boltzman Law s = 5.67x10-8 W/m2/K4 A is area in m2 T in Kelvins
Example: The Sun
L = 5.7x10-8 x 4 x 3.14 x (7x108m)2 x (5500K)4 = 4 x 1026 W
4x1026 Watts = 100 billion billion MegaWatts!!

26. Question
If you were to double the temperature of the Sun without changing its radius, by what factor would its luminosity rise? 2 4 8 16 32

27. Answer
If you were to double the temperature of the Sun without changing its radius, by what factor would its luminosity rise? 2 4 8
16 = 24 32

28. Emission Lines Electron Drops Energy Levels of H Photon Escapes
Can Only Happen Between
Certain Pre-determined orbitals
Spectrum of Hydrogen
Each Element Has Different Orbitals
So Each Element Has Different Lines

29. Absorption Lines Light moving through cold
gas can have photons removed.
Creates dark wavelengths
called absorption lines

30. Question
A star is viewed through a far away hydrogen gas cloud, what kind of spectrum can we expect to see? A) Absorption only
B) Emission only C) Continuum only D) Emission and Continuum E) Absorption and Continuum

31. Answer
A star is viewed through a far away hydrogen gas cloud, what kind of spectrum can we expect to see? A) Absorption only
B) Emission only C) Continuum only D) Emission and Continuum E) Absorption and Continuum

32. Stars Come in Different Colors

33. Stellar Temperature Stars come in different sizes and temperatures.
Can the two be linked?