max asked in Science & MathematicsPhysics · 1 month ago

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

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  • NCS
    Lv 7
    1 month ago
    Favourite 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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