What is exactly a blueray dvd?

blueray disks hold MUCH more info than a normal dvd, it's also in a different format. It's basically NOT a dvd at all.
 
It is a DVD player that uses a blue laser to read the disc. Since blue light is shorter wavelength than red, it can be focused into a smaller spot. This allows it to read the tracks and pits on the disc that are closer together. Placing the pits closer together allows more data to be packed onto the disc.
The same thing was done with the transition from CD to DVD. But in that case it was from far infrared to red lasers.

BTW: as to cost, Blue lasers are expensive.
 
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