MOD 5   LECTURE 6                                                DIFFRACTION

In this lecture you will:

SUMMARY OF WAVE - MATTER INTERACTIONS
Type of waves Main interaction  SOUND LIGHT (EMS)
INTERFERENCE Run into each other acoustics (loud/soft) holograms (bright/dark)
REFLECTION Hit and bounce off surfaces acoustics: echoes and reverberations mirrors, colors, shiny and matte
REFRACTION Enter clear materials and bends rumble of thunderstorms lenses, diamonds, rainbows
DIFFRACTION Bend around solid or little slit how we hear around a post slits, rainbow effect on CD or DVD

 
SOUND DIFFRACTION
This is what happens when waves hit a solid object and  bends around it.  It is similar to the way waves of water hit the opening in breakers.  

When sound hits a small object, or goes thru an opening the waves bend.  What happens later is the waves may run into each other and interfere, but the BENDING is due to DIFFRACTION. 

This is why sound can be heard in another room thru the door without standing right in front of it, the waves of sound bend around the opening. 
 


 

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


 
LIGHT DIFFRACTION
When there is a narrow opening light moves as a wave, hits the narrow opening and bends around it which produces a fuzzy image.  
"In his 1704 treatise on the theory of optical phenomena (Opticks), Sir Isaac Newton wrote that "light is never known to follow crooked passages nor to bend into the shadow". He explained this observation by describing how particles of light always travel in straight lines, and how objects positioned within the path of light particles would cast a shadow because the particles could not spread out behind the object."

"On a large scale, this hypothesis is supported by the seemingly sharp edges of shadows cast by rays from the sun. However, on a much smaller scale, when light waves pass near a barrier, they tend to bend around that barrier and spread at oblique angles. This phenomenon is known as diffraction of the light, and occurs when a light wave passes very close to the edge of an object or through a tiny opening, such as a slit or aperture. The light that passes through the opening is partially redirected due to an interaction with the edges."

Hold a razor blade or a sewing needle to light and you can see the waves bend when coming through the slit opening, or, project that onto paper.  

- diffraction grating separates light into spectrum
 

"In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in monochromators and spectrometers."

Your spectrometer (spectrophotometer, spectrograph or spectroscope) uses a diffraction grating to split light into the visible spectrum.  


Diffraction grating seen on cd or dvd

 
SUMMARY OF EFFECTS
loud and soft areas of sound in concerts sound interference
bright or dim areas lined up in a row on a surface light interference
holograms  light interference using lasers
reverberation sound reflection
echoes sound reflection
mirrors light reflection
colors  light reflection and absorption
shiny or matte light reflection and interference
soap bubbles and oil spill rainbows light reflection and interference
the rumble of thunder sound refraction
prism rainbow light refraction
rainbows light refraction and reflection
diamonds twinkle light refraction and reflection
blinding early morning light  reflection less than critical angle 50o
hearing around corners diffraction
spectroscopes diffraction
rainbow effect on DVDs diffraction
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