Reflection and Diffraction (1 Viewer)

[Petyo]

Can any please explain the differences between those two?
As far as I know, 'diffraction' means bending or spreading out of waves around an obstacle-but is "diffraction" simply a change in direction of waves through a specific angle? If so, is it also "refraction"?

 

[Tully B.]

I got the impression that while refraction changes the direction of light (eg when light passes through a piece of glass), diffraction occurs either when light bends around an obstacle, or when it spreads out through an aperture. Reflection is also the change in direction of light, though under different circumstances as refraction (ie bouncing of a mirror - law of reflection).

 

[Aerath]

Been a while since The World Communicates but I'll have a go:
Reflection occurs when the angle of incidence equals angle of refraction, ie the beam of light stays in the same medium. (Wikipedia: Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. )
Diffraction refers to the spreading out of waves when it passes through a small aperture, ie, light and sound going through a doorway. Diffraction is significant only when the width of the aperture is comparable or smaller than the wavelength of the incident wave. For insstant, there is no diffraction of visible light through a doorway, because the width of the doorway is very much greater than the wavelength of light (like 5x10^-7m). However, for like sound waves, its wavelengths are like 880 000m long, therefore, it diffracts more. That's why when you're standing 'around the bend', sound waves travel quite far, however, light waves don't.

 

[Petyo]

@@
you know the Bragg's X-ray diffraction experiment? Some books use the term "Bragg reflection"...

 

[Tully B.]

Yeah, where he used crystals as diffraction gratings for x-rays, which allowed them (Bragg + son) to measure specific length of various x-rays, as well as determine the atomic structure of specific crystals.

 

[Petyo]

so 'diffraction' is similar to 'reflection' ?@@

 

[rama_v]

so 'diffraction' is similar to 'reflection' ?@@

Firstly, diffraction: To think of it you might like to look at the Huygen's-Fresnel construction. Basically, consider a spherical wave propagating radially outwards. Each spot on the wave front acts as a source of secondary waves. From this construction it is easy to see how diffraction occurs, as the waves passing through a small slit will spawn secondary waves from every spot of the wavefront, resulting in wave vectors adding and subtracting at various areas. It can get quite mathematical, but fundamentally it's a pretty simple idea.
Go here for more.

Huygens–Fresnel principle - Wikipedia, the free encyclopedia

Secondly, 'Bragg Reflection' is sometimes used because you can imagine the light as a beam which reflects off of the planes in the crystal lattice. Of course, as they reflect they are interfering constructively and destructively with one another, producing a diffraction image that is indicative of reciprocal space. From the reciprocal space, it is possible to reconstruct the crystal lattice structure that produced the 'Bragg reflection' (i.e. diffraction pattern) image.How? Each spot will indicate a set of planes satisfying the Bragg condition. You can go from there...

 

[Aerath]

Do we need to know about the Huygens Fresnel thing? =\
I've never heard of it.... =\

 

[rama_v]

Do we need to know about the Huygens Fresnel thing? =\
I've never heard of it.... =\

No, it's not in the sylabus.

But if it helps you understand why things occur then you should learn it