1.a) Neil A. Armstrong was the first person to walk on the moon. The distance be
ID: 1550291 • Letter: 1
Question
1.a) Neil A. Armstrong was the first person to walk on the moon. The distance between the earth and the moon is 3.85 × 108 m. Find the time it took for his voice to reach the earth via radio waves. (b) Someday a person will walk on Mars, which is 5.60 × 1010 m from the earth at the point of closest approach. Determine the minimum time that will be required for a message from Mars to reach the earth via radio waves.
2.The maximum strength the magnetic field in an electromagnetic wave is 8.9 × 10-6 T. What is the maximum strength of the electric field of the wave?
3.A certain type of laser emits light that has a frequency of 6.4 × 1014 Hz. The light, however, occurs as a series of short pulses, each lasting for a time of 3.1 × 10-11 s. (a) How many wavelengths are there in one pulse? (b) The light enters a pool of water. The frequency of the light remains the same, but the speed of the light slows down to 2.3 x 108 m/s. How many wavelengths are there now in one pulse?
Explanation / Answer
1
a) 3.85 X 10^8meters / 2.99 X 10^8 m/sec = 1.29 seconds
b) 5.60 x 10^10 / 2.99 x 10^8 m/sec = 187 seconds = 3.12 minutes
2. E/B = c or E = c*B = 3*10^8*8.9*10^-6 = 2670N/C
Maximum electric field strength in the wave = 2670 N/C or V/m
3 the number of wavelengths in a pulse must remain the same as it enters the water. In fact, when a ray of light travels into water, the frequency remains the same, as the question states, the speed and the wavelength become directly proportional to one another, so a reduction in speed results in a corresponding reduction in wavelength, so the wavelength becomes shorter but the frequency, and number of pulses per wavelength, remain the same. So the answer to (b) must be the same as the answer to (a),
(a). The number of wavelengths per pulse is equal to the length of the pulse divided by the time period of the pulse. If we let T be the length of the pulse in seconds and t be the period of a single wave, that means the number of wavelengths per pulse is T/t. But by definition t = 1/f, the period of the wave is the inverse of the frequency, so that means the number of waves per pulse is
Tf
= (3.1 x 10^-11 s)(6.4x 10^14 Hz) = 19840 = 1.98 x 10^4, and the answer to (b) is the same.
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