Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.
Doppler effect astronomy questions?
Q: In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm. Express each answer in three significant digits.
1.) Find the speed for a star in which this line appears at wavelength 120.7 nm.
For this one, I used the doppler formula and got -2220 km/s or -2.22x10^3 but the answer was 2200 Km/s.
2.) Find the speed for a star in which this line appears at wavelength 121.0 nm.
For this one, I used the doppler formula and got -1421 km/s or -1.421x10^3 but it was wrong and I don't know the answer.
3.) Find the speed for a star in which this line appears at wavelength 121.8 nm.
4.)Find the speed for a star in which this line appears at wavelength 122.4 nm.
- NCSLv 73 months agoFavorite Answer
v/c = (λshift - λrest) / λrest
1) v / 3e8 = (120.7 - 121.6) / 121.6 → → v = -2.22e6 m/s
but they asked for the SPEED which is always positive; 2220 km/s should be an acceptable answer
2) By the same means, I get 1480 km/s.
3) 493 km/s
4) 1970 km/s
You seem to have the correct method; your error was in misinterpreting the word "speed."
If you find this helpful, please select Favorite Answer!