Hi Carlos,
In his book "Crystals, X-rays and Proteins", Dennis Sherwood
explained in the first chapter: why do use x-rays?
Using his analogy: for a small boat (5 m length) in the ocean,
waves come in from the ocean with a wavelength (say 20 - 30 m) are
merely pass underneath the boat.
On the other hand, shorter waves (surface ripples produced by
throwing stones) is reflected by the boat. In other words, an
observer (who can't see the boat but can detect the waves),
will record the reflection of the short wavelength ripples, but the
long waves pass on undisturbed. So, using the long waves will not
help to detect the boat!
From the above example we get the idea that to study an object
by means of its effect upon wave motions, we must choose a wave form
with a wavelength smaller than, or comparable, to a linear dimension
of that object. Now, applying this idea to crystals should give
insights to our original question "why x-rays??". Crystals have two
significant linear dimensions: the apparent dimension of the crystal
(say 1 mm) and the atomic scale dimensions in the crystals (say 0.15
nm). Using a visible light of wavelength 600 nm (yellow light) [about
1,666 times smaller than the macroscopic dimension of the crystal but
about 4000 times greater than the atomic scale dimension. The light
will see the crystal as a whole but not its atomic details. However,
using X-rays (wavelength of the order of 0.1 nm) should be affected
by the atomic scale structure of the crystal; hence, information
about the atomic structure can be obtained using the X-rays.
Ibrahim
At 04:09 AM 1/24/2007, you wrote:
Hi,
I have once heard and recently read that "the diffraction event
results from the fact that both the X-rays wavelength and the atomic
distances are of the same magnitude". Although such a relation seems
appealing I am unsure if this is not a mere coincidence. Could
someone clarify or lead me to a relevant reading.
Cheers,
Carlos
--
**************************************
Dr. Carlos Frazao
Crystallography Department
ITQB-UNL, Av Republica, Apartado 127
2781-901 Oeiras, Portugal
Phone: (351)-214469666
FAX: (351)-214433644
e-mail: [EMAIL PROTECTED]
www.itqb.unl.pt
Ibrahim M.Moustafa, Ph.D.
Pennsylvania State University
Biochemistry & Molecular Biology Dept.
201 Althouse Lab.
University Park, PA16802
Tel (814) 863 8703
Fax (814) 865 7927
