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1. What is the frequency of a "dental" x-ray having a wavelength of 0.1Å? What is its
energy?
0.1 Å = 0.1x10-10 m = 10-11 m

c


3x10 8 m / s
10 11 m
 3x1019 Hz
E  h  (6.63x10 34 Js)(3x1019 Hz)  2x10 14 J
2. The star Zeta Puppis has a temperature of 30,000K. At what wavelength is it
brightest?

3x10 7 ÅK 3x10 7 ÅK

 1000 Å
T
3x10 4 K
3. A white dwarf star has a radius of 7x10 cm and a temperature of 10,000K. What is
its luminosity? Compare to the Sun.
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7x108 cm = 7x106 m
A  4r 2  4 (7 x10 6 m) 2  6.2 x1014 m 2
L  AT 4  (5.7 x10 8W / m 2 K 4 )(6.2x1014 m 2 )(10000 K ) 4  3.5x10 23W
Lsun is 4x1026 W, so this is about 0.001 times the Sun’s luminosity
4. A radio transmitter on a spacecraft emits a signal at a frequency of 10 Hz. At Earth
the signal is received and noted to be at 99,970,000Hz. How fast is the spacecraft
moving? Is it receding or approaching?
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v 

c 0
   0  99,970,000 Hz  108 Hz  30,000 Hz
vc

 30,000 Hz
 3x10 8 m / s
 9 x10 4 m / s
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0
10 Hz
receding because frequency decreased. The minus sign isn’t important as long as you
know how to tell whether it’s approaching or receding.
5. The star Vega has a parallax of 0.14". How long will it take light to travel there and
back?
Round trip takes 42 years.