Absorption line, wavelength.?

Question

In the spectrum of a galaxy an absorption line of hydrogen is observed at a wavelength of 494.9nm. In the laboratory the wavelength of the same line is found to be 490nm. assume the speed of light is 300,000km/sec and explain this and derive what information you can from these facts. No matter how much i try to understand this question, it has me stumped!! xx

Answer 1

It's obviously a homework question - so I'm only giving hints...

You need to use the Doppler effect.

Basically the galaxy is moving away from us, and so the wavelengths get stretched (so get longer).

You should have an equation that relates the change in wavelength (i.e. observed wavelength minus the rest frame / lab wavelength) to the speed of the galaxy divided by the speed of light.

So you can say tha galaxy is moving away from us and you can calculate its radial velocity.

Answer 2

There is a change in wavelength of 4.9nm. The galaxy is a longer wavelength because it is moving away from you.

If you take a rifle and shoot at a target every second a bullet will hit the target every second.

If you are on a truck moving away from the target at one foot per second, every bullet will have to travel another foot more than the previous bullet. the spacing is greater. the frequency has decreases.

The same thing with the specific wavelength of light. as the galaxy moves away, it's wavelength (the spacing between a given point of the cycle, let;s say the peak) gets longer.

The speed of light divided by wavelength equals the frequency of light. The frequency has decreased, giving you a shift toward the red end of the spectrum.

Answer 3

This is actually a much more complex problem than the previous respondents let on. The obvious conclusion is that the light from the other galaxy has become reddened somehow, with a slightly longer wavelength, assuming that hydrogen out there is the same as hydrogen down here. Hubble thought this was a Doppler effect due to a receding velocity. However, he has since been proven wrong. There is no recession velocity, only an illusion of recession velocity. The current wisdom is that all of the galaxies are "at rest" with respect to each other and it is the Universe itself and the space in between the galaxies that is expanding, which rules out a Doppler effect. So what the problem is really asking you is how much has the fabric of space stretched in the time since that light left that galaxy, and you are not given enough data to answer that question. If they gave you the Hubble constant then you might take a stab at it. I hope this helps.

P.S. I'm surprised that Angela S. doesn't know this.