# a question about Bragg's experiment

Discussion in 'Physics' started by logicman112, Oct 19, 2010.

1. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
2*d sin(teta) = n * lambda
This is the formula of Bragg. How can we measure 'n'?
An important fact which no physic book wants to talk about!!
'n' is the order of the reflected beam, what does it mean? How we can measure it?

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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If none of 'em talks how did u know abt it?

Apr 20, 2004
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4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Do you mean this?

Bragg's Law

*************

Curses! Beenthere and moved on!

5. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
Last edited: Oct 25, 2010
6. ### Papabravo Expert

Feb 24, 2006
10,340
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Your mistake is the expectation that there is a single solution -- there isn't. The statement that "n is an integer" just means that there may be multiple solutions depending on the wavelength of the light and the geometry of the crystal lattice. Why are you being so obstreperous anyway?

7. ### DeminJanu New Member

Oct 26, 2010
3
0
as stated above, there are many solutions.
every time 2*d*sin(theta) equals an integer multiple of the wavelength, you see a bright spot. ie. you see a whole set of bright spots, equally spaced apart.

similar to this:
http://en.wikipedia.org/wiki/File:Single_slit_and_double_slit2.jpg
(slightly different math, but same idea of coherent interference from regularly spaced sources/reflection)

8. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
I thought Bragg's experiment can measure the wavelength. So it can not!!
Because there are two variables unknown, n and the wavelength!!

So why the following extract from a famous physic book states that William Bragg invented this theory to measure wavelength?
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In 1913, William Bragg and his son Lawrence, proposed a simple method of
measuring the wavelengths of X-rays by means of their reflection from crystal planes.
Crystals comprise atoms (or ions) that are arranged in a spatial lattice, such that they....
---------------------------------------------------------------------------------------

Last edited: Oct 31, 2010
9. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
Please add your comments then I will let you know something which has never been mentioned in physic books. How 'n' can be measured in Bragg's experiment logically?

10. ### Markd77 Senior Member

Sep 7, 2009
2,803
594
Am I missing something? I'd have thought you increase the angle from zero until you see a patch of X-rays. That means n=1. The next patch is n=2, etc.
I imagine you see X-rays at angle = 0 because they go through the sample - ignore that.

logicman112 likes this.
11. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
Mark, you are right.
We see the first bright spot in the minimum path difference and the minimum path difference happens when n=1.