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thodge5
2008-Oct-10, 06:03 AM
I have some questions and I hope you're willing to help. I have provided that answer I chose below, please give me your thoughts. Thanks to those who give me feedback.

1. The observed spectral lines of a star are all shifted towards the red end of the spectrum. Which statement is true?
a. The star is not rotating.
b. This is an example of the photoelectric effect.
c. The star has a radial velocity away from us.
d. The second law of Kirchhoff explains this.
e. The star has a radial velocity towards us.

I chose E. I believe that if the star has a radial velocity that is
approaching the viewer, the Hydrogen alpha will measure at 655.0 nm
and sit at the "red side" of the spectrum.

2. An incandescent light (glowing tungsten filament) produces:
a. a continuous spectrum, with the peak giving the temperature of the
filament.
b. a continuum, with dark lines of tungsten and argon as well.
c. an emission spectrum, with bright lines due to ionized tungsten.
d. a continuum, with bright tungsten lines added.
e. an absorption spectrum, with dark lines due to the solid filament.

I chose A. But i am uncertain. We used small spectrum films in class and
he turned the light on. I believe I saw the complete array of the
spectrum, but he mention something about additional lines, I didn't
understand his statement.

3. Emission lines of hydrogen that are found in the ultraviolet part of the electromagnetic spectrum are formed by electrons transitioning from:
a. level 2 to any level
b. any level to level 3
c. level 1 to any level
d. any level to level 2
e. any level to level 1

I chose E. According to the Lyman series, any transfers to level 1 produce
energy and frequency that is equivalent to ultraviolet. But the text shows
a double-sided arrow from any level to level 1, and vice versa, that is
also ultraviolet. Which would also make answer C right.

4. True/ False: The red hydrogen alpha line carries more energy per photon
than the blue-green hydrogen beta line does.

False: after doing research I read that the hydrogen beta emits more
energy in the form of blue-green. Also, according to the spectra blue/
green emits a higher frequency, meaning more energy per photon.


5. The Orion Nebula, M-42, is a hot, thin cloud of glowing gas, so its spectrum is:

a. a few bright lines against a dark background.
b. a few dark line in the continuum.
c. a continuum, but with both bright and dark lines mixed in.
d. a continuum, strongest in the color red.
e. not in the visible portion of the spectrum.

I chose A. According to Kirchoff's Laws, a low-density hot gas produces
an emission spectrum, which correlates to the elements present in the
studied object. Regardless of the element(s), the spectrum would be a
few bright lines amongst a black background.

thodge5
2008-Oct-10, 06:08 AM
I am looking for some guidance, this subject is difficult for me. I'm studying hard, and would really appreciate any different explanations in relation to the topic. thank you.

PraedSt
2008-Oct-10, 08:40 AM
More homework! Well, at least you're having a go this time. How did you do in the last ones?

For these, I make your score 2.5/5.
Although, for the ones you have right, I'm not sure you've understood the physics. They may be lucky guesses :)

Looking up these topics might help:

Q1 Doppler shift, Red shift
Q2 Incandescence, Black body radiation
Q3 Emission spectrum, Energy levels
Q4 Colour, frequency, photons
Q5 Orion Nebula, Emission spectrum, Absorption spectrum

Personally I'd wait for a BAUT expert to provide better answers.

Cougar
2008-Oct-10, 03:06 PM
I am looking for some guidance...

Here is my guidance: Do the reading.

Neverfly
2008-Oct-10, 03:12 PM
thodge5
Given the answers you provided, I would say that (right or wrong) you should believe in yourself a bit more...
You can handle it.

Remember back when Arithmetic was hard and 4th graders moaned and groaned about long division?
You've got it- So run with it.;)

Ken G
2008-Oct-10, 07:58 PM
You made a go so I'm happy to give you the answers:

1. I chose E. I believe that if the star has a radial velocity that is
approaching the viewer, the Hydrogen alpha will measure at 655.0 nm
and sit at the "red side" of the spectrum. Redder corresponds to lower frequency. Ask yourself, does the sound of a race car engine sound higher pitched (higher frequency) or lower pitched (lower frequency) when it is approaching you? This is also an example of the Doppler shift.


2. I chose A. But i am uncertain. That is correct, but the rest of your answer sounds like you aren't sure why. A tungsten film emits "blackbody radiation", so you don't get "lines" from it. You need to find out what that statement means.


3. I chose E. According to the Lyman series, any transfers to level 1 produce
energy and frequency that is equivalent to ultraviolet. But the text shows
a double-sided arrow from any level to level 1, and vice versa, that is
also ultraviolet. Correct, but again, be sure you know why. C is not correct because of the key word emission in the question. Which causes emission, and which causes absorption, in terms of transitions into and out of level 1?


4. False: after doing research I read that the hydrogen beta emits more
energy in the form of blue-green. Also, according to the spectra blue/
green emits a higher frequency, meaning more energy per photon.
Correct, but be aware that the color all by itself tells you this-- blue photons always have more energy than red ones, it makes no difference what their source is.


5. I chose A. According to Kirchoff's Laws, a low-density hot gas produces
an emission spectrum, which correlates to the elements present in the
studied object.I suspect that is what the question is driving at, yes, although there will also be continuum emission in the infrared from dust thermal emission and in the visible from light scattered by dust-- the gas regions won't be perfectly dust free. That's why I never give true/false questions, more knowledge can sometimes mess you up and you have to try hard not to over-analyze. I think the idea of the question is that most of the dust is concentrated in dust lanes and we are imagining that the gas regions are pretty pristine, so it's all emission lines.

So I score you 4/5, I'm not sure why other scores have not been as high but they may have deducted points for your explanations, which do indeed reveal some problems.

thodge5
2008-Oct-10, 08:03 PM
I've done all the reading. I used the 40 pages of lecture notes, the assigned text book chapter, and alternate resources to find the answers.

On the previous, I received a 7 out of 10.

I'm not just looking for the answers, I seriously want a 10 out of 10 but also understand that I need to know the material for the Midterm and Final Exam. And the professor rambles on miscellaneous topics and it is hard to comprehend.

cjl
2008-Oct-10, 08:28 PM
KenG's answers are excellent, and a good reference for what you should be looking for. Here's how I would do these:



1. The observed spectral lines of a star are all shifted towards the red end of the spectrum. Which statement is true?
a. The star is not rotating.
b. This is an example of the photoelectric effect.
c. The star has a radial velocity away from us.
d. The second law of Kirchhoff explains this.
e. The star has a radial velocity towards us.

I chose E. I believe that if the star has a radial velocity that is
approaching the viewer, the Hydrogen alpha will measure at 655.0 nm
and sit at the "red side" of the spectrum.

I'm not sure where the hydrogen alpha line comes into this - it is really just a simple doppler shift question. If all of the spectral lines are shifted towards the red, then they are all at a lower frequency (and therefore lower energy) than they were initially, implying a radial velocity away from the observer (us).

Think of sound - if a car is coming towards you, its engine sound is higher in frequency, and if it is receding from you, its engine sound is lower in frequency. The same applies to light, though much higher velocities are required for a noticeable effect.



2. An incandescent light (glowing tungsten filament) produces:
a. a continuous spectrum, with the peak giving the temperature of the
filament.
b. a continuum, with dark lines of tungsten and argon as well.
c. an emission spectrum, with bright lines due to ionized tungsten.
d. a continuum, with bright tungsten lines added.
e. an absorption spectrum, with dark lines due to the solid filament.

I chose A. But i am uncertain. We used small spectrum films in class and
he turned the light on. I believe I saw the complete array of the
spectrum, but he mention something about additional lines, I didn't
understand his statement.

You are correct here - a simple blackbody glowing will show a continuous spectrum that is only dependent on temperature.



3. Emission lines of hydrogen that are found in the ultraviolet part of the electromagnetic spectrum are formed by electrons transitioning from:
a. level 2 to any level
b. any level to level 3
c. level 1 to any level
d. any level to level 2
e. any level to level 1

I chose E. According to the Lyman series, any transfers to level 1 produce
energy and frequency that is equivalent to ultraviolet. But the text shows
a double-sided arrow from any level to level 1, and vice versa, that is
also ultraviolet. Which would also make answer C right.

Again, you are correct with E. C is incorrect because you have to think of whether energy is gained or lost with any particular process. If the lines in question are emission lines, is the gas gaining or losing energy?




4. True/ False: The red hydrogen alpha line carries more energy per photon
than the blue-green hydrogen beta line does.

False: after doing research I read that the hydrogen beta emits more
energy in the form of blue-green. Also, according to the spectra blue/
green emits a higher frequency, meaning more energy per photon.

Correct, though make sure you don't overthink this. All you need to know is that H-beta is blue-green, and then you have all the information you need. As you correctly stated, the fact that bluish green light is higher frequency means more energy per photon.



5. The Orion Nebula, M-42, is a hot, thin cloud of glowing gas, so its spectrum is:

a. a few bright lines against a dark background.
b. a few dark line in the continuum.
c. a continuum, but with both bright and dark lines mixed in.
d. a continuum, strongest in the color red.
e. not in the visible portion of the spectrum.

I chose A. According to Kirchoff's Laws, a low-density hot gas produces
an emission spectrum, which correlates to the elements present in the
studied object. Regardless of the element(s), the spectrum would be a
few bright lines amongst a black background.
While there are some additional factors that could weigh in, the answer the question seems to be aimed towards is A. Since it explicitly states that the nebula is a hot thin cloud of gas, you are right to then correlate that to an emission line spectrum.

Overall, I would say you aren't doing too bad, and your main problems lie in a lack of confidence, and overthinking the problems. Try not to overanalyze, and instead look for the basic concept the problem is trying to test you on.

PraedSt
2008-Oct-10, 08:33 PM
On the previous, I received a 7 out of 10....And the professor rambles on miscellaneous topics and it is hard to comprehend

7/10..nice one.
You seem to have a nightmare prof. I've had that a couple of times, and it's not pleasant.
So sorry, marking was unnecessarily mean! As Ken says, I deducted marks for explanations...and unsystematically at that, which makes me a bad marker.
I have a small suggestion. Others may not agree, but I've found solving numerical problems to be a better way to understanding physics. So try those instead of MCQs?