Date: Tue, 27 Apr 2010 13:18:42

Author: Bernard Cleyet

Subject: Re: DVDs as gratings?


Try a CD.

bc still finds the vast majority of DVDs sold are single layer.

p.s. Just tried a DVD; no prob. The CI and the first orders (+ & -) of a diode LASER are on the wall.

Make your own spectroscope:

p.p.s. I just tried a DVD-RDL which also works well. I got this pattern:

X x x CI x x X

with a green LASER muiltiple x's still only two X's

Also I found both are sl. transparent, i.e. work as transmission gratings.

On 2010, Apr 27, , at 11:29, Dr. Richard Tarara wrote:

> My last lab of the semester is about wave optics and after using a calibrated grating to find the wavelength of our lasers, we go on to do a bunch of other measurements, the last being finding the physical length of the track on a CD. That can be easily done using the CD as a reflection grating and then some simple geometry. Thought I'd try a DVD this year, but can't get a diffraction pattern that is usable. Get a center spot and then a fairly distant skewed line, but only on one side of the center spot. Can get some small diffraction paterns around the central spot. Figure one problem (perhaps THE problem) is that most, if not all, DVDs are now dual layer--so we have two gratings, one above the other and even worse--much of the light is going through the first layer--being diffracted, going into the second layer, being diffracted, reflected back to the first layer..... So I'm guessing the macroscopic pattern is just being destroyed because you don't have the right conditions for reinforced constructive interference. Tried a data DVD thinking it might not be dual layer--same result--but then thought that the initial recording is probably in the outer layer with considerable light (obviously our laser is broad and not focussed at the data depth) passing through to the reflective layer, then back out.
> Does any of this make sense? Has anyone successfully produced diffraction patterns from DVDs or BlueRays? What's causing the small diffraction pattern near the center (reflected) spot?
> Rick
> ***************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN
> ******************************
> Free Physics Software
> New multi-input energy simulator--plays similar to the Montana State analog computer.
> *******************************

From Tue Apr 27 16:36:08 2010