# Optical Drive Physics

1. Jul 18, 2012

### guppster

1. The problem statement, all variables and given/known data

1. 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?

2. Why will a silver mirror not show a reflection when you sand the surface with very coarse sandpaper?

2. Relevant equations

3. The attempt at a solution

2. 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?

2. Jul 19, 2012

### cepheid

Staff Emeritus
For all of these optical drives, a laser is used to write data to the disks and then read it back from it. I think some well-placed Googling can get you pretty far on this question.

As for the other question, your answer is basically correct. Look up diffuse reflection vs. specular reflection. If the surface is perfectly smooth (compared to the wavelength of the light), then incoming rays that are all parallel will be reflected all parallel as well. So an image of the scene in front of the surface is faithfully reproduced. This is specular reflection. If the surface has all kinds of "canyons and valleys" in it, then the angle of incidence for different parallel rays is different, because they all strike different parts of the surface that are oriented differently. So, even if these rays are parallel initially, they won't be reflected parallel to each other, but instead in all different randomized directions. This is diffuse reflection, and it is the way that light is reflected from the vast majority of objects that you see on a day to day basis.

3. Jul 19, 2012

### Simon Bridge

4. Jul 19, 2012

### guppster

Thank you cepheid and Simon Bridge!