Download Physics 127 Descriptive Astronomy Homework #3 Key (Chapter 2

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Physics 127 Descriptive Astronomy
Homework #3 Key (Chapter 2)
Winter 2015
2-1. For each of the following wavelengths, state whether it is in the radio, microwave, infrared,
visible, ultraviolet, X-ray, or gamma-ray portion of the electromagnetic spectrum and explain
your reasoning: (a) 2.6 μm, (b) 34 m, (c) 0.54 nm, (d) .0032 nm, (e) 0.620 μm, (f) 310 nm, (g)
0.012 m.
In all cases my reasoning is based on the information on wavelength ranges for various spectral regions given in Figure
2-6 of the textbook. The answers are (a) 2.6 μm  infrared, (b) 34 m  radio, (c) 0.54 nm  X-ray, (d) 0.0032 nm 
gamma-ray, (e) 0.620 μm  visible, (f) 310 nm  ultraviolet, (g) 0.012 m  microwave. (3/7 pts. per answer. Please
round total to neares 0.1 pt.)
2-2. A cellular phone is actually a radio transmitter and receiver. You receive an incoming call in the
form of a wave of frequency 880.65 MHz. What is the wavelength (in meters) of this wave?
Using the equation f = c/ found in the second column of p. 38 of the text, we can solve for the wavelength . Here
 = c/f = (2.998108 m/s) / (8.8065108 /s) = 0.0.340 m = 34.0 cm.
2-4. If you double the Kelvin temperature of a hot piece of steel, how much more energy will it
radiate per second?
The Stefan-Boltzmann Equation (p.43) tells us that the energy radiated per second from a black body is proportion to
its temperature in Kelvins raised to the fourth power. We expect our piece of steel to radiate approximately as would a
black body. Therefore its radiative output would increqse by a factor of 24 = 16.
2-5. The bright star Bellatrix in the constellation Orion has a surface temperature of 21,500 K. What
is its wavelength of maximum emission in nanometers? What color is this star?
From Wien’s law, found on p.42 of the text, max = 0.0029 K m/T = 0.0029 K m/21,500 K = 1.3510-7 m = 135 nm.
Bellatrix’s color is bluish-white.