# a question about Bragg's experiment

#### logicman112

Joined Dec 27, 2008
69
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?

#### R!f@@

Joined Apr 2, 2009
9,758
If none of 'em talks how did u know abt it?

#### Wendy

Joined Mar 24, 2008
22,708
Do you mean this?

Bragg's Law

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

Curses! Beenthere and moved on!

#### Papabravo

Joined Feb 24, 2006
17,629
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?

#### DeminJanu

Joined Oct 26, 2010
3
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)

#### logicman112

Joined Dec 27, 2008
69
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?
---------------------------------------------------------------------------------------
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:

#### logicman112

Joined Dec 27, 2008
69
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?

#### Markd77

Joined Sep 7, 2009
2,806
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.