Chapter 25
8.
Picture the Problem
: This is a units conversion problem
Strategy:
Use the appropriate unit conversions to measure the distance of one light year in kilometers,
the speed of light in light years per year, and the speed of light in feet per nanosecond.
Solution:
1. (a)
Express 1
ly in km:
2.
(b)
Write the speed of
light in lightyears per
year:
3. (c)
Convert the speed
of light to units of feet
per nanosecond:
Insight:
In solving problems involving the light year it is helpful to remember that the unit ly is a unity
of distance, where
the speed of light times one year.
12.
Picture the Problem
: The wavelength of light from a star that is moving toward Earth is Doppler
shifted to a shorter wavelength.
Strategy:
Replace the frequency in equation 253 with the wavelength given in
equation 254. Solve the resulting expression for the shifted wavelength divided by
the unshifted wavelength.
Solution:
1
. (a) Because the star is moving toward Earth, the frequencies of the electromagnetic waves
are increased. However, the measured wavelengths are less than what they would be if the star were at
rest relative to us because the wavelengths are inversely proportional to the frequencies
2. (b)
Write the Doppler shift in terms of
wavelength:
3.
Solve for the ratio of wavelength:
Insight:
The wavelength is shorter than it is when the star is at rest relative to Earth, as predicted.
25.
Picture the Problem
: Dental Xrays have a wavelength of 0.30 nm.
Strategy:
Solve equation 254 for the frequency.
Solution:
Calculate the frequency:
Insight:
This frequency falls in the Xray part of the spectrum as described on page 848 of the text.
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 Spring '08
 Dr.G
 Physics, Light, Wavelength

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