Integrated Coordinated Science for the 21st Century
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Answers
Physics To Go
- The speed of light is too large for Galileo to have measured. If the hilltops were a few hundred meters apart, the travel-time for light is only about one millionth of a second. Human reaction time is at least a half a second. The only delay Galileo was able to observe was due to the reaction time of his assistant.
- a) The estimate for the wavelength is 1.0 X 10-6 m. In fact, wavelengths of visible light range from about .40 X 10-6 m to .75 X 10-6 m.
b) From c = λ f, the frequency is 3 X 1014 Hz.
- a-b) See chart below.
AM radio:
FM radio/commercial TV:
radar:
microwaves:
infrared radiation:
light:
ultraviolet radiation:
x-rays:
gamma rays: |
3 x 102 m
3 m
0.3 m
0.03 m
3 x 10-4 m
5 x 10-7 m
3 x 10-8 m
3 x 10-10m
3 x 10-13 m |
- See chart below.
From—To
Earth to Moon
Earth to Sun
Sun to Pluto
Sun to nearest star |
light travel-time
1.2 s
500s = 8.3 min
2.0 x104 s = 5.6 hr
1.4 x 108 s = 4.3 light-years |
Note: The number of seconds in a year is 60 X 60 X 24 X 365 = 3.2 X 107 s. To find the travel-time to the nearest star in light-years, divide 1.4 X 108 s by 3.2 X 107 s.
- a) 1.6 X 10-5 s
b) No. The travel time was much too short.
- Not necessarily. If extraterrestrials could see microwaves, their eyes would be much larger than ours.
- We can choose a frequency that penetrates space well, like microwaves. We can choose a frequency that is near (or perhaps a multiple of) 1420 MHz, which is the frequency of microwave radiation given off by the hydrogen atom.
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