Matter/Electron Waves and the Davisson-Germer Experiment

[Yosty22]

Homework Statement

This is a conceptual question that I am not quite sure about. In the Davisson-Germer Experiment (details here), do the electron diffraction peaks that occur only show up if the incident beam of electrons hits the crystal surface perpendicularly?

Homework Equations

The Attempt at a Solution

I was really confused when reading this in my textbook when it relates to this question. The book talks a lot about Davisson and Germer's initial intent which was to "attempt to understand the arrangement of atoms on the surface of a nickel sample by elastically scattering a beam of low-speed electrons from a polycrystalline nickel target. (If you are unfamiliar with the story, someone dropped a flask of liquid air on the vacuum system, oxidizing the nickel target. They then realized that the crystalline structures it created scattering of electrons, showing that the electron is also a matter-wave).

However, in my textbook, they talk about \alpha being the target orientation, and that is 90°. After that brief mention, they do not really speak of it again. Because of this, I am a little unsure as what to believe. Would these peaks arise if the angle was different than 90°?

 

[BvU]

Wikipedia has this 90° in its story too. But they refer to Bragg's law for the angles. There the pictures have angle of incidence = exit angle. I think perpendicular incidence isn't essential, but it gives a nice expression for angles of constructive interference.

 

[Yosty22]

That is what I thought as well. I thought that the 90 degree angle makes it "easier" - like the simplest case of what actually happens. However, I did not know if there was an actual reason why it does or does not matter.

Thank you.

 

[gracy]

What is glancing angle?
I found this definition but I am not getting this.
the angle between a ray incident on a plane surface and the surface,as of a beam of electrons incident on a crystal;the complement of the angle of incidence.

 

[BvU]

You found what definition ? Why isn't this question in a separate thread ?
http://en.wikipedia.org/wiki/Angle_of_incidence#Grazing_angle