Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: hamza on 05/10/2007 05:17:08
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I was wondering the other day that which phenomenon of light causes that colour display on our Compact Discs. Is it interference or diffraction. Please give the reason also.
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It is a combination of the two:
Diffraction is the tendency of the edges of a wavefront to radiate circularly so light can go around a corner
[diagram=281_0]
In a cd the millions of pits will have millions of edges so the light will diffract from all of them.
Interference is how two or more light waves interact with one another. If they are moving in time they will add if moving out of time then subtract. So the diffracted light interferes to produce bright and dark patches.
Different colours have different wavelengths so their bright and dark patches occur in different places so you see different colours in different places.
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I was wondering the other day that which phenomenon of light causes that colour display on our Compact Discs. Is it interference or diffraction. Please give the reason also.
It's called interference by diffraction grating. You can see the same effect on butterfly's colours, for example.
http://en.wikipedia.org/wiki/Diffraction_grating
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It is a combination of the two:
Diffraction is the tendency of the edges of a wavefront to radiate circularly so light can go around a corner
[diagram=281_0]
In a cd the millions of pits will have millions of edges so the light will diffract from all of them.
Interference is how two or more light waves interact with one another. If they are moving in time they will add if moving out of time then subtract. So the diffracted light interferes to produce bright and dark patches.
Different colours have different wavelengths so their bright and dark patches occur in different places so you see different colours in different places.
Daveshorts! i did not actually get u.. Where does the light difract from. I mean there are no slits on a cd. Light cannot pass through it, so howcome it gets bended. I would really appreciate if u could eloborate it to make it clear to me.. Thanx
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Diffraction doesn't only occur at slits. When light encounters a sharp corner, it will "diffract" (bend around the corner). A CD's surface consists of many tiny bumps, all of which have sharp edges. Each of these edges will bend the light so that when it reflects off the CD it interferes as Daveshorts said.
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I mean there are no slits on a cd. Light cannot pass through it, so howcome it gets bended. I would really appreciate if u could eloborate it to make it clear to me.. Thanx
The dots and dashes ("pits") on a CD are all cut in a very long spiral groove running from the centre of the disk to the outer edge. The gap between each spiral turn is constant, so from the centre to the edge you have a large number of uniformly-spaced and almost-parallel "scratches" on the surface. This is then effectively a diffraction grating.
There's a nice undergraduate experiment where you measure the wavelength of laser-light using a common steel ruler! ... You put the ruler on the table and shine the laser at a very shallow glancing angle on the half-millimetre rulings on the ruler - and you get a diffraction pattern of spots on the wall. Measure the spacing of the spots and apply the formula - and you've measured the wavelength of the light (about 600nm, or 0.0006mm for a red laser). You could try this with a laser-pointer.
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The word 'diffraction' refers to what happens when a wave strikes any object, hole or boundary. It is a very general term.
'Interference' usually refers to the effect of waves combining from a number of discrete sources - say slits, holes or radio antennae. It is a specific example of diffraction and is usually easier to calculate because it consists of a simple addition of the waves rather than a difficult mathematical integration. (loosely speaking)
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Hamza,
this experiment was written by Dave, although you can do it with your CD as well. I know you like experiments :-)
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/why-sodium-street-lights-make-things-look-orange/