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# G485 June 2012 Paper and unofficial mark scheme

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1. Well sorry...
2. (Original post by joefoxon)
My working, for rikhilrai.

The wavelengths given are the one of viewing the galaxy from the Earth, and one from viewing the galaxy from just next to the galaxy. You need to use the one from next to the galaxy (480), because from that point of view, the light would not be redshifted. By the time it reaches the Earth, there will be a shift.
I disagree, part d was no longer considering two individual galaxies. It was considering planet Earth and the a given galaxy at a distance from Earth.

The 480 was from the galaxy moving away from Earth.
3. I'll just re-read the question again, because I think I've realised something.

EDIT: Yeah, the source was on the Earth, so you are right... That's a shame, ah well.
4. (Original post by joefoxon)
I'll just re-read the question again, because I think I've realised something.

EDIT: Yeah, the source was on the Earth, so you are right... That's a shame, ah well.
It was a really poorly worded question and I had to go over that question several times after you mentioned your suggestion.
5. (Original post by joefoxon)
My working, for rikhilrai.

The wavelengths given are the one of viewing the galaxy from the Earth, and one from viewing the galaxy from just next to the galaxy. You need to use the one from next to the galaxy (480), because from that point of view, the light would not be redshifted. By the time it reaches the Earth, there will be a shift.
Are you stuuupid?

Firstly, what you are implying is that the light from galaxy has got shorter i.e. Blueshifted. Since this only happens for one galaxy out of billions, the andromeda galaxy, this seems pretty unlikely.

Secondly, your thing about the viewing from the galaxy is ridiculous. Even if we had some telescope next to the galaxy (a galaxy that is so far away even the dinosaurs couldnt have reached it if they set off at the speed of light (which they can't, yes basic physics but i feel the need to say this)), the data sent back to us on earth from the telescope would be red shifted itself, so your point would still be wrong.

So the answer is to use the shorter wavelength.

End of.
6. (Original post by Dorkus Maximus XII)
Are you stuuupid?

Firstly, what you are implying is that the light from galaxy has got shorter i.e. Blueshifted. Since this only happens for one galaxy out of billions, the andromeda galaxy, this seems pretty unlikely.

Secondly, your thing about the viewing from the galaxy is ridiculous. Even if we had some telescope next to the galaxy (a galaxy that is so far away even the dinosaurs couldnt have reached it if they set off at the speed of light (which they can't, yes basic physics but i feel the need to say this)), the data sent back to us on earth from the telescope would be red shifted itself, so your point would still be wrong.

So the answer is to use the shorter wavelength.

End of.
Cool story bro
7. (Original post by Dorkus Maximus XII)
Are you stuuupid?

Firstly, what you are implying is that the light from galaxy has got shorter i.e. Blueshifted. Since this only happens for one galaxy out of billions, the andromeda galaxy, this seems pretty unlikely.

Secondly, your thing about the viewing from the galaxy is ridiculous. Even if we had some telescope next to the galaxy (a galaxy that is so far away even the dinosaurs couldnt have reached it if they set off at the speed of light (which they can't, yes basic physics but i feel the need to say this)), the data sent back to us on earth from the telescope would be red shifted itself, so your point would still be wrong.

So the answer is to use the shorter wavelength.

End of.
Actually, when I noticed that my answer would give a blue shift, I looked at the question again and found what I got wrong.

The question says that we are viewing the Earth from that galaxy, so how is that any different to what I said, other than that you're looking the opposite direction?
8. (Original post by joefoxon)
My working, for rikhilrai.

The wavelengths given are the one of viewing the galaxy from the Earth, and one from viewing the galaxy from just next to the galaxy. You need to use the one from next to the galaxy (480), because from that point of view, the light would not be redshifted. By the time it reaches the Earth, there will be a shift.
nvm, someone already pointed it out
9. (Original post by joefoxon)
Well sorry...
i wasnt insulting you i was meaning if you had interpreted it right then the question was really badly worded :L

(sorry for the confusion )
10. (Original post by just george)
i wasnt insulting you i was meaning if you had interpreted it right then the question was really badly worded :L
Ahhh, fair enough then :P By the looks of things, I was wrong, but I still have no idea exactly what the question is saying.
11. (Original post by joefoxon)
Ahhh, fair enough then :P By the looks of things, I was wrong, but I still have no idea exactly what the question is saying.
Yeah it is very odd, im not convinced it really makes sense the way theyv put it tbh :L but oh well, hope for your sake they dont penalise using 480nm instead of 393.4.. otherwise i guess its only 1 mark
12. (Original post by just george)
Yeah it is very odd, im not convinced it really makes sense the way theyv put it tbh :L but oh well, hope for your sake they dont penalise using 480nm instead of 393.4.. otherwise i guess its only 1 mark
how can they NOPT penalise for that?????

if you used that value it just proves that you have no idea what your doing......
redshift = stretching(expansion) of space

if a light wave STRETCHES the wavelength STRETCHES which means it is bigger.
how could you use the smaller value for the STRETCHED OUT value
13. Because it seemed to say that the wavelength of the light observed coming from the galaxy was 480, so I thought that was the unshifted light. As soon as I realised that it would be a blue shift, I changed my mind.
14. They were asking you to find the speed of the recessing galaxy given that a particular wavelength gets redshifted by whatever it was. And hence find the distance using hubbles equation
Not hard to understand? :L
15. (Original post by just george)
Yeah it is very odd, im not convinced it really makes sense the way theyv put it tbh :L but oh well, hope for your sake they dont penalise using 480nm instead of 393.4.. otherwise i guess its only 1 mark
I imagine it'll score at least 1 mark, 2 if they're feeling really generous. It was towards the end of the exam, so people are rushing and silly mistakes are more frequent.
16. (Original post by jamesvernon)
They were asking you to find the speed of the recessing galaxy given that a particular wavelength gets redshifted by whatever it was.

Not hard to understand? :L
What the question said was that there was a calcium source on Earth, and another calcium source in a distant galaxy. If you look at the source on Earth, it won't be redshifted, because you're next to it, but if you look at the one in the galaxy, it will be redshifted, because it's a long distance away and moving.

I FINALLY GET IT!
17. (Original post by joefoxon)
What the question said was that there was a calcium source on Earth, and another calcium source in a distant galaxy. If you look at the source on Earth, it won't be redshifted, because you're next to it, but if you look at the one in the galaxy, it will be redshifted, because it's a long distance away and moving.

I FINALLY GET IT!
They could have used anything, including just telling you there was a difference in light. Instead they threw in some synoptic stuff to see who remembers. I don't get why everyone is having a problem understanding this!
18. I think they deliberately tried to catch people out with it. Seems to have worked
19. (Original post by zabuzar)
how can they NOPT penalise for that?????

if you used that value it just proves that you have no idea what your doing......
redshift = stretching(expansion) of space

if a light wave STRETCHES the wavelength STRETCHES which means it is bigger.
how could you use the smaller value for the STRETCHED OUT value
(Original post by Joseppea)
I imagine it'll score at least 1 mark, 2 if they're feeling really generous. It was towards the end of the exam, so people are rushing and silly mistakes are more frequent.
Sorry, but i didnt realise his method got blueshift, i though he just used deltalambda/lambda = v/c, but used the value 480 for lambda instead of 393.4. Effectively like how you calculate percentage error, where they allow both answers. If thats all that has been done wrong, i would think it would be 2/3 atleast... otherwise iv misunderstood what the error was
20. (Original post by just george)
Sorry, but i didnt realise his method got blueshift, i though he just used deltalambda/lambda = v/c, but used the value 480 for lambda instead of 393.4. Effectively like how you calculate percentage error, where they allow both answers. If thats all that has been done wrong, i would think it would be 2/3 atleast... otherwise iv misunderstood what the error was
Don't worry I did the same thing, didn't even think - just instinctively put the largest number as the denominator.

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Updated: June 15, 2012
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