In hydrogen, the transition from level 2 to level 1 has a rest wavelength of 121.6 nm.1).Find the speed for a star in which this line appears at wavelength 120.4 nm .2）.Find the direction for a star in which this line appears at wavelength 120.4 nm .toward us or away from us?3).Find the speed for a star in which this line appears at wavelength 121.4 nm .4).Find the direction for a star in which this line appears at wavelength 121.4 nm .toward us or away from us?5).Find the speed for a star in which this line appears at wavelength 122.2 nm .6）.Find the direction for a star in which this line appears at wavelength 122.2 nmtoward us or away from us?7).Find the speed for a star in which this line appears at wavelength 122.9 nm .8.Find the direction for a star in which this line appears at wavelength 122.9 nm .toward us or away from us?

1).

2). Toward us

3).

4). Toward us

5).

6).  Away from us

7).

8). Away from us

Explanation:

Spectral lines will be shifted to the blue part of the spectrum if the source of the observed light is moving toward the observer, or to the red part of the spectrum when it is moving away from the observer (that is known as the Doppler effect).

The wavelength at rest is 121.6 nm ()

Then, for this particular case it is gotten:

Star 1:

Star 2:

Star 3:

Star 4:

Star 1:

Toward us

Star 2:

Toward us

Star 3:

Away from us

Star 4:

Away from us

Due to that shift the velocity of the star can be determine by means of Doppler velocity.

 (1)

Where is the wavelength shift, is the wavelength at rest, v is the velocity of the source and c is the speed of light.

(2)

Case for star 1 :

Notice that the negative velocity means that is approaching to the observer.

Case for star 2 :

Case for star 3 :

Case for star 4 :

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