Question

CDs and DVDs are read optically using lasers. A typical laser used to read a compact disc emits photons of wavelength ~670 nm (in the red region of the visible spectrum) and has 1.0 mW power.

How many photons are emitted by this laser per second?

Blu-ray devices (i.e. blue laser diodes, based on GaN) are becoming more popular than DVDs for optical data storage.   What is the advantage of using shorter wavelength lasers for this purpose?

Answer 1

The energy of a photon is given by the equation E = hc/λ, where E is the energy, Planck's constant h = 6.63 x 10^(-34) (with nasty units), c = 3.00 x 10^8 m/s (speed of light) and λ is the wavelength of the light (in metres).

So in your case, λ = 670 x 10^(-9), so the energy of one photon is:

E = 2.9686 x 10^(-19) Joules.

1 mW means 0.001 W, which is 0.001 Joules per second. So the number of photons released per second is 0.001 / (2.9686 x 10^(-19)) = 3.368 x 10^15.

Advantage of Blu-Ray.
1. Huge storage capacity. Although Blu-ray can’t quite fit an entire series of HD-quality material on one disc, it could potentially fit an entire series of standard DVD quality stuff on one. That’s pretty good, considering the storage savings alone.
2.Mandatory Managed Copy. If you haven’t heard of Digital Rights Management (DRM) before, well, then this is a really good time to be check it out. DRM is the copyright protection scheme the media industry uses to prevent piracy, and the Blu-ray’s technology in this realm is actually quite exciting. The possibility exists for users to copy the content of a disc a limited number of times, similar to Apple’s iTunes system.
3. Backwards compatibility. hough not forced down any throats, the Blu-ray Disc Association is encouraging manufacturers to make the players fully backwards compatible. That will allow users to both read and write on CDs, DVDs, and, obviously, Blu-ray discs.
4. Quality support Sony and Philips might be the strongest backers of Blu-ray, but other major corporations have announced future plans to support the technology. Some of these include Apple, Dell and Panasonic.