Diffraction pattern due to a cd

[captainabhi]

In the project "Using a laser pointer to measure the data tracks spacing on CDs". Due to presence of what? does diffraction pattern occurs & what kind of diffraction pattern occurs?


pls can any one of you experts tell me the Answer as i will be asked about this in my Viva Voce.... it's kinda big deal for me...pls help


Q1 in experiment, which part of the cd serves as a single slit: due to which diffraction pattern occurs?
Q2 why can't we use equation of single slit hoffard diffraction but Only BRagg's Equation??



PLs help.....

[wendellwiggins]

Hello captainabhi,

Q1: No part.
Q2: The single slit equation pertains to a single slit.

The CD diffraction experiment does not have a single slit nor does it transmit light. It has tracks (many, many of them) and each one reflects light. The composite light from all the reflections forms a diffraction pattern.

Many different arrangements of reflecting and transmitting devices produce many different diffraction patterns. If you are still interested in the CD experiment, please submit a relevant question. Or if you are interested in the single slit experiment, pose a question about it.

Good luck, WW

[captainabhi]

Thank you WW,
for your answer, but still i have queries.

Q1.Can you explain why we are using the equation "d(sin θm - sin θi ) = mλ". in this diffraction pattern.& the logic behind it?

Q2 Can you please elaborate the process, which takes place when we shine the laser pointer to a CD.

Thank you for your help.

[wendellwiggins]

captainabhi,

Q1.Can you explain why we are using the equation "d(sin θm - sin θi ) = mλ". in this diffraction pattern.& the logic behind it?
This equation comes from considering the angles at which the light scattered from the collection of CD tracks adds together constructively. The light adds constructively if the path for the light reflecting off one track is a multiple of wavelengths different from its neighbors. A web search for "diffraction patterns" will turn up several discussions for this equation or equations like it for slightly different arrangements of diffractors.

Q2 Can you please elaborate the process, which takes place when we shine the laser pointer to a CD.
The laser illuminates many of the tracks on the CD, each of which scatters some of the light into a range of angles. The scattered light adds up constructively at some angles and destructively at other angles. The material given in the "Background" section of the project description goes into this in more detail.

[captainabhi]

but why don't we observe maxima as all difracting patterns are of equal intensities in the project?

[wendellwiggins]

captainabhi,

The diffraction pattern from the CD has VERY strong maxima at the angles indicated by the formula. Each maximum is so strong, you likely don't notice anything else.

Since you seem to be interested in understanding diffraction, you need to read about it in depth. I cannot provide a complete tutorial on all the aspects. I suggest you read some of the materials already on the web, or better yet, get a textbook that describes optical diffraction in detail.

WW

[captainabhi]

Thank you , WW that clears a lot.