An electromagnatic wave is traveling in vacuum with frequency 7.4 x 10 14 Hz. Th
ID: 2036533 • Letter: A
Question
An electromagnatic wave is traveling in vacuum with frequency 7.4 x 1014 Hz. The wave has average total energy density of 4.8 x 10-6 J/m3. (a) What is the speed of this wave?
v =______ m/s
(b) What is the wavelength of this wave?
l = _______m
(f) How long does it take for this wave to travel from the sun to the earth?
t =_______ min
(g) How many wavelengths of the wave fit in this distance?
N = wavelengths
A police car on the side of the road (at rest) uses a radar gun to catch speeders as they approach it. The frequency the radar gun emits is 8 x 109 Hz and the speed limit is 65 mi/hr. What is the difference in frequency of the emitted and returned radar wave? (A positive answer means the returned wave has a greater frequency than the emitted wave; a negative answer means the returned wave has a smaller frequency than the emitted wave.)
freturned - femitted = ______Hz
Explanation / Answer
Given that,
f = 7.4 *10^14 Hz
(a)
speed of this wave,
v = 3*10^8 m/s
(b)
wavelength of this wave,
l = v / f = 3*10^8 / 7.4*10^14
l = 0.405*10^(-6) m
l = 4.05*10^(-7) m
(c)
Time taken to travel from the sun to the earth,
t = d / v
t = 15*10^10 / 3*10^8 = 500 s
t = 8.20 min
(d)
number of wavelengths of the wave fit in this distance,
N = d / l
N = 15*10^10 / 0.405*10^(-6)
N = 3.7*10^17
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