# @spath, etc. A question about Pit-light Destructive Reflection

Since a CD is made so that a pit sticks out 1/4 the length of the laser length in polycarbonate (about 500 nm), or 125 nm. The offset of this delays the light reflection by 1/2 the wavelength, cancelling out the light of the pit, making the pit appear dark.

How is it insured that, due to inconsistencies in the media, the laser perhaps is hitting only the pit. How is this compensated for?

Is this only “corrected” by C1?

Could someone also explain to me the laser optics system if the first paragraph is false somehow?

There is no special trick to ensure this, just quality of the surfaces and the accuracy of the mechanisms.

The servo process is well explained at http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm

Ideally, half of the spot is on the pit, and half is expanding around, and the two halves cancels themselves. If I understood well the above paper, this is the smallest size the spot can have, when it is perfectly focused. The shape of the spot will change if the CD is uneven or if the lens moves. This is detected and corrected in real time by four detectors, before it’s too late.

Now, a little riddle :
What becomes of the energy of the beam, that was emitted by the laser, if the light is canceled ?

Heat maybe?

No, the shape of the surface doesn’t make it absorb more light.

A Clue : look at any CD (exept black ones), you can see it.

The light isn’t really cancled, it’s just made to appear dark, since the light is delayaed half a wavelength. It is mutually destructive.

Nonsense ! The destruction is effective, the level, as well as the energy is zero.

Answer : the light is diffracted.

The mistake is to consider the process from the point of view of Descartes’ optics, that are no longer accurate when we reach the wavelenght scale itself. The phenomenon is explained by Huygens’ optics.
The light is reflected in all directions, but, at an ordinary scale, the waves cancel themselves in all drections but the reflexion angle, that is the only direction where there is light.
But on a CD, the spot is very small, and the light is cancelled in the direction it should reflect, but not in some other directions. Those directions depends on the wavelenght, that is the color of the light.

So the cancelled light become rainbows !

The rainbows you can see on a CD are all the light that was canceled in the direction it should have been reflected. It has not disappeared, nor has it turned into heat, it has just been diffracted.
On some phtalocyanine CDs (Ricoh, Mitsui), you can even see that the rainbows are brighter than the direct reflect of a lamp. You can also see on non engraved parts the full reflect of the lamp, very much brighter than on the groove.

If you’ve got a red (or any color) laser toy, you can also point it to a CD, and see several reflections at different angles (that are the angles where the red part of the rainbow would appear).