Optical Drive Physics: Data Storage & Reflection

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Discussion Overview

The discussion revolves around the physics of data storage and retrieval in optical devices such as CDs, DVDs, and Blu-ray discs, as well as the optical properties of reflective surfaces, particularly in relation to how surface texture affects reflection.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant inquires about the differences in data storage methods among optical devices and the physics behind reflection on a silver mirror when its surface is sanded.
  • Another participant suggests that the lack of reflection from a sanded mirror is due to increased surface scattering, which disrupts clear reflection.
  • It is noted that optical drives work by creating pits in the disk's surface to represent binary data, with the depth of these pits being measured by lasers.
  • The amount of data that can be stored on a disk is influenced by factors such as material quality, encoding methods, and the wavelength of light used for reading and writing data.
  • A later reply mentions that diffraction from the pits on a CD surface contributes to the colors observed when white light is shone on it.

Areas of Agreement / Disagreement

Participants generally agree on the basic principles of how optical drives function and the effects of surface texture on reflection, but there is no explicit consensus on the finer details of these processes or the implications of the observations made.

Contextual Notes

The discussion includes assumptions about participants' prior knowledge and does not resolve the complexities involved in the physics of optical data storage or the optical properties of surfaces.

Who May Find This Useful

This discussion may be of interest to those studying optics, data storage technologies, or anyone curious about the physical principles underlying everyday optical devices.

guppster
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I have been recently wondering how is data stored on and retrieved from optical devices like CDs, DVDs, and Bluray. What makes these different storage types different from each other?

Why will a silver mirror not show a reflection when you sand the surface with very coarse sandpaper? Is it because the coarse sandpaper will make small dents in the material, so the light will reflect off the surface in multiple directions, making it hard to see a clear image or is it because of another reason?
 
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150 views is actually quite a lot of interest - however, the questions involve things that are covered at a very junior level in school and, anyway, are easy to google for yourself. So you'd get a better response if you included your attempts to find out with specific questions about what you don't understand.

However, it is more likely that people are just getting back to you ... it can take days to get a reply: there's ages to go before we can safely conclude that there is little interest in the question. You got an answer to the other thread while I was writing this for instance. (Posting two threads with the same subject can reduce your response rate too.) Since you generally write good questions I'd be more patient if I were you :)

You showed you had a go thinking through the mirror one - so I'll do that first.
The common household mirror has a sheet of glass in front of the main reflective surface. At the front surface of the glass there are three main things happening to the light - 1. reflection, 2. transmission+refraction, 3. surface scattering

For the mirror to work you need 1 and 3 to be small so the reflection off the back surface dominates. Roughing the surface up makes 3 dominate. These processes are general to all surfaces. So your intuition there was good.

But do you see how basic that description had to be - it is hard to describe without sounding patronizing. Of course you know there's glass in front of a mirror!

Optical drives are quite complex in the details but the basic concept is to burn little pits in the metal surface in the disk. The pits are positioned to represent 1s and 0s. The depth of the pits is measured using LED lasers. The amount of data you can fit on a disk depends on the quality of the materials, the encoding (which may include data compression) and the wavelength of the light used to burn/read the pits. (Shorter the wavelength the closer together the pits can be, the more data can be stored in the same place.) I assume here that I don't need to explain how lasers can measure distance or how data can be stored as 1s and 0s?

See also:
http://www.opticsetc.com/how-does-an-optical-drive-work.html
http://en.wikipedia.org/wiki/Light_scattering
 
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Thanks for the reply Simon Bridge!
Yeah, I had already understood the mirror question, but your last paragraph has really helped me :)
 
It's all good then :) the link has a much more detailed description.
In fact - diffraction from the pits in the surface is what causes the colors you see when you shine white light on a CD.
 

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