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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. 8 7x108 cm = 7x106 m A 4r 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? 8 v c 0 0 99,970,000 Hz 108 Hz 30,000 Hz vc 30,000 Hz 3x10 8 m / s 9 x10 4 m / s 8 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.