# Crystal Grating

## Main Question or Discussion Point

The atoms in a metal (ex. Cu) are arranged as a 3-D grating. But to our common sense ,smooth metal surfaces only reflect lights. Why can't we see diffraction from metal surfaces?

Related Other Physics Topics News on Phys.org
abbas_majidi
The metals contain a see of free electrons. In EM we know refractive index for metals is complex. Imaginary part of n is responsible for absorption and reflection of EM waves. This sea is like a cover which is in surface of metals. Therefore the wave cannot see grating.

Why can't we see diffraction pattern from crystals without see of free electrons like dielectrics?

Since from formula of diffraction sin(Teta)=i*landa/d must d>landa( <=> sin(Teta)<1) and in crystal the d(distance between two atoms) is about angstrom therefore there is no diffraction pattern for ultraviolet up to radio waves. for example we don't see light diffraction from glasses.

X-ray and gamma have diffraction through crystals and X-ray is one of must important instrument in crystallographic.
Also in metals we have X-ray diffraction. Gamma and X-ray can pass from surface and see grating. This lead to have diffraction from metals.

I apologize for scribbling English. My language isn't English but i am trying to improve it.

abbas_majidi
I make mistake in above,difference of real part of n and the environment refractive index is responsible of reflection.

The metals contain a see of free electrons. In EM we know refractive index for metals is complex. Imaginary part of n is responsible for absorption and reflection of EM waves.
What do you mean for "imaginary part of n is responsible for absorption and reflection of EM waves" ?
My native language isn't English too.

abbas_majidi
In Electromagnetic theory refractive index of metals(n) obtain as complex number and imaginary part of n arises from absorption of EM waves by metals. In other word complex n is Representative of absorption in matter.