Optical Drive Physics: Data Storage & Reflection

[guppster]

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?

 

[Simon Bridge]

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

 

[guppster]

Thanks for the reply Simon Bridge!
Yeah, I had already understood the mirror question, but your last paragraph has really helped me :)

 

[Simon Bridge]

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.