HW 5-4 Answers
(140) 10, 11; Supplemental Questions 6 and 7
RQ10: How can the Doppler effect
explain wavelength shifts in both
light and sound?
Both light and sound are waves. Waves of
any sort, if they come from a moving source,
will always be compressed ahead of the
source and stretched out behind the source.
Friday, October 26, 12
HW 5-4 Answers
Star
(140) 10, 11; Supplemental Questions 6 and 7
Star’s velocity
RQ11: Why does the Doppler effect detect only
radial velocity?
Line of Sight
Earth
Friday, October 26, 12
The Doppler Effect only works for “towards” or
“away” velocities. Only this type of motion will
cause compression or extension of the
wavelength.
Friday, October 26, 12
Friday, October 26, 12
HW 5-4 Answers
SQ6! Look at the figure at the top left of page 147. We measure
the Balmer beta lines (H") for three stars get this data:
! ! Star 1! 486.3 nm!
Star 2! 486.5 nm!
Star 3! 485.8 nm
From the figure: λo=486.1 nm
Star 1 is 0.2 nm longer
(a) Find the difference between these
away
red
values and the accepted value.
(b) For each star determine if it’s
moving towards us or away from us
and …
(c) … if it is red shifted or blue shifted.
Star 2 is 0.4 nm longer
away
red
Star 3 is 0.3 nm shorter
toward
blue
(d) Rank these stars from slowest to
fastest.
Star 1 is slowest, then 3, and Star 2 is fastest.
Friday, October 26, 12
HW 5-4 Answers
SQ7
You're driving along and see a red bumper-sticker that says "If this
sticker is blue, you're driving too fast." Your kid sister says, "What does
that mean?" How do you explain it to her?
If you are quickly approaching an object, it should be blue-shifted,
which could make the red bumper-sticker appear blue.
In reality, you would need to be traveling about one-third the speed of
light, which isn’t reasonable. That’s why it’s a joke.
{The Doppler Shift is based on
relative speeds, so the bumpersticker should read, “…you’re
driving too much faster than
me!”}
Friday, October 26, 12