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Phy 3031 Modern physics, Spring 2008
Homework set #9. Due at the start of class on Monday, March 3, 2008.
For numerical answers give two significant digits. The first should be correct, the second can
be approximate.
1. The cosmic microwave background radiation fits the black-body distribution for a
temperature T = 2.7 K.
(a) What is the wavelength of the maximum intensity in the spectrum?
(b) What is the frequency of the radiation at the maximum?
(c) What is the total power incident on the Earth from this background radiation?
2. A 40 W incandescent light bulb radiates from a filament at a temperature T = 3300 K.
Assume that the filament radiates like a black body.
(a) What is the wavelength of the radiation at the maximum in the spectrum?
(b) What is the frequency of the radiation at the maximum?
(c) Assume that λm is a good estimate of the average wavelength of the photons
radiated. About how many photons does the filament radiate every second?
(d) If you are standing 5 m away and are looking at the filament, then how many
photons enter your eye every second? Assume that the diameter of your pupil is
5 mm.