# 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.

### 1 Answer

- NCSLv 71 month agoFavourite 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."

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