January 24 Through February 4—TUCSON, ARIZONA: Annual show
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 Post subject: How do you find the log from the chatroom
PostPosted: Fri Feb 23, 2007 9:52 pm 
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Hi,

I've been trying to find the logs from the last two weeks tutoring sessions,
http://yey.be/chat logs/chatlog75.log, and the next I suppose would be chatlog76.

I tried the forum search engine, and even google,and couldn't find them.

Anyone know where I should be putting the above address?

Thanks,
Love gemmsong

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PostPosted: Fri Feb 23, 2007 11:20 pm 
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Doos would be the one to ask.


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PostPosted: Sat Feb 24, 2007 1:10 am 
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Gosh, I didn't look for it yet. I sure hope he didn't forget to post it. :?

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PostPosted: Mon Apr 02, 2007 11:35 am 
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Hi all,

Due to summer/winter time changes, we adjusted the Saturday chat times a little.
11 PM London, 3 PM Los Angeles, 6 PM New York, 12 PM Paris and 8 AM Melbourne (Sunday).

So one hour later for people in the northern hemisphere and 1 hour earlier for those who are used to walk upside down.

Further more I made a (weak) commitment to be in the chat daily from around 5PM London time for any questions (that would be around 11AM NY time) to about 7PM London time.


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 Post subject: Guest chat
PostPosted: Mon Apr 02, 2007 7:00 pm 
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Hi all,

Mervyn L. Cohn of http://www.e-praise.com/ has agreed to hold a guest chat with us next Saturday (April 7th) at 7 PM San Francisco time (that is 10 PM New York, 3 AM London (Sunday), 4 AM Paris (Sunday) and noon Melbourne (Sunday)).
Follow the blue link for international times.

Besides being an avid member of this forum, Mr. Cohn is also an ASA senior appraiser and a certified gemologist. And then of course he also finds the time to develop and run e-praise.com. Through the years Mr. Cohn has gathered a tremendous amount of knowledge and he is willing to share some of that with us.

He doesn't step away from a good discussion and (or but) I'm sure that after next Saturday you will agree with me that he has a wonderful mind and is a joy to talk to.

We haven't really set a topic, but I can imagine us talking a lot about diamonds during the session. Please all clear your schedule for this one.

p.s.: the regular Saturday chat (spectroscope) is canceled and we will resume the week after.


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PostPosted: Wed May 16, 2007 2:24 pm 
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Hi,

Last weekends chat is posted at http://yey.be/chatlogs/chatlog84.cgi . It was about advanced use of the polariscope (part of the sequel) and how to determine optic sign with it. We also learned some nice physics from Brian, so I was lucky to sit back and listen/learn.
Google for Malus' Law for the calculations.

I want to do a special on how to discriminate natural from synthetic amethyst/citrine with the aid of a polariscope, but that will have to wait for another week until I have some pictures of the typical Brewster fringes. A very interesting topic.

Anyone have a topic for next week?


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PostPosted: Thu May 17, 2007 5:08 pm 
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Hi Doos,

Well if we are following the syllabus then we still have the second half of the PPL chapter to finish...pleochroism...Differential selective absorbtion and Dichroscope.

I'm happy to do any other subject though and come back to this if any of the others have a topic they'd like to go through???...If you have guys then post before Saturday so poor Doos and Annie can have a chance to do a bit of prep work...then think up hard questions :twisted:

See you all Saturday

Be well

Frank


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PostPosted: Fri May 18, 2007 3:42 pm 
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Doos wrote:
Hi,

We also learned some nice physics from Brian, so I was lucky to sit back and listen/learn.
Google for Malus' Law for the calculations.



Ah Doos, don't be so bashful, you knew all that stuff I was describing. I certainly don't recall mentioning Malus' Law by name :wink:


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PostPosted: Fri May 18, 2007 3:55 pm 
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Brian,

Although I knew some parts of what you were describing, it took someone with a clear mind on the topic to make me really understand it.
All an engine needs is a spark and for that I must thank you.


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PostPosted: Fri May 18, 2007 11:39 pm 
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Wow, the chat log was really a nice format tfor reviewing discussion. I admire the effort each of the participants puts forth in these discussions. Self-teaching physics to each other is not an easy task, especially for the leaders of the discussion group. But I have witnessed it to be a worthwhile effort. Occasionally I track when my students work with each other in this sort of round-table discussion; it is always clear when that happens, because exam scores increase a letter grade.

I'll correct a point in my discussion: there is always a temptation to confuse circular polarization with optical activity. And sometimes confusion strikes even the expert, especially when he is wholly unprepared to discuss the subjects. So, what is the difference between the two??

Well, first, what is similar between the two? We define PPL as light linearly polarized along some direction in the plane perpendicular to propagation. Then both phenomena describe the rotation of the PPL around that plane, like the revolution of an analog clock's second hand.

However, there is significant differences between the two. The first difference is that circular (or elliptical) polarization is a condition that doesn't depend upon the medium through which the light travels (thus, can exist as light travels through empty space), while optical activity occurs only as light travels through specialized media such as quartz.
The second difference is the distance- (or time-) scale required to make one complete rotation of the polarization vector. Circular polarization = rotation on unimaginably-short length scale, whereas optical activity = rotation on a human length scale.

Circular polarization takes some effort to set up. Passing through a 1/4 wave filter, a plane-polarized light wave transitions to circularly (or elliptically) polarized, and continues on that way through empty space or air or whatever not-special medium you have in mind. But most important, the rotation of the of the polarization vector is very fast, so that one complete 360 degree (or 2pi radian) rotation occurs each wavelength distance the light travels (a complete rotation in less than a micrometer).

Optical activity, in contrast, only occurs as light traverses specialized materials (such as quartz), or... (and here's where my interest lies) if a material is located in a strong magnetic field (in which case, it is referred to as "Faraday effect"). So optical activity doesn't occur in every medium through which light can traverse. But also, in order to rotate the polarization vector through a measureable angle (like say 2 or 3 degrees), the light to travel distances much larger than its wavelength... on the order of centimeters to meters. So you could be talking tens of meters or greater to achieve a complete 360 degree (pr 2pi radian) rotation.

So, I incorrectly posted in the tutor discussion: optical activity is NOT HIGHLY dependent on distance traveled through the medium.

And, oh yah, just ignore the spaghetti commentary... I must've been hungry at that point.

But other than that, reading through the transcripts of my deposition, I don't think I composed any other off-the-cuff lies. Most notably, optical activity IS dependent upon wavelength, which is why you get such cool images with vegetable oil through the crossed-polars.


***I dunno, this license to spout random physics may turn out to be a bad thing: it can be so difficult to turn off the extremely-long-winded, takes-up-all-the-time-between-classes professor in me :(***


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PostPosted: Sat May 19, 2007 11:03 am 
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PostPosted: Sat May 19, 2007 12:08 pm 
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Just look for the bullseye, MoDo. :wink:
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PostPosted: Sat May 19, 2007 12:18 pm 
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Hi Modo and JB,

The problem with reading the chatlogs is that if you are not there, it is just one big block of text. That makes it almost impossible to read and stay focused.
If you are there, things get more easier and you have time to ask questions.


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PostPosted: Sat May 19, 2007 12:29 pm 
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I didn't read the chatlogs but, I assume the question was, "What causes the bullseye optic figure."


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PostPosted: Sat May 19, 2007 12:35 pm 
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Hi,

I would say "optical activity" or enantiomorphism.

The chats are not the same without you two, so if you have time today, please join in, it starts at 6pm Eastern.


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