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Uploaded:2015-11-10
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Blu-rays can hold about ten times more than DVDs because Blu-ray players use special blue lasers to read them. But it took a while for scientists to figure out how to make those lasers work.

Hosted by: Michael Aranda
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Sources:

Basics on Blu-Ray & Optical Discs
http://www.blu-ray.com/faq/
http://www.explainthatstuff.com/cdplayers.html

Optics
http://www.edmundoptics.com/technical-resources-center/imaging/wavelength-effects-on-performance/
http://voer.edu.vn/m/aberrations/986b67d8

On Nakamura
http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/nakamura-facts.html
http://www.npr.org/templates/story/story.php?storyId=5488821
https://goo.gl/LkpI82
[Intro plays; Text: QQs: Why can Blu-rays hold more than DVDs?

Micheal: Say you've just finished work on your first multi billion dollar 3D Hollywood blockbuster. All the fancy lighting, celebrities, explosions, and romance gets distilled down into one big movie file, or the way a computer sees it, one long string of ones and zeros. Now you want your movie to be on a disk, meaning all those ones and zeros need to be stored in some kind of physical form, like a DVD or Blu-Ray.

But your movie file is really big, it's really long and super high definition, so you've run out of room on a normal DVD which only holds 4.7 gigabytes of data. Well, you could always try putting it on a Blu-Ray. Since Blu-Ray players use a higher energy blue laser, then can fit much more data, up to 50 gigabytes, on one disk.

On both types of disks, all the zeros are stored by  carving little tiny pits into the disk, all the ones are stored by not cutting pits in what are called lands. When it's time to watch your movie, the player shines a laser, a super-concentrated beam of light emitted by electrons that absorb extra energy, then release that energy in the form of light. The laser goes through a lens and then focuses on the disk. Your player detects whether the light hit a land or a pit depending on how the beam is reflected or scattered, and interprets that as either a one or a zero.

But how much data you can put on the disk depends on the laser. The beam needs to be narrow enough to detect the pits and lands without accidentally reading two at the same time. And that's where color comes in. Every color of light has its own wavelength, and the smaller the wavelength, the smaller you can focus a laser beam of that color and the smaller the pits and lands you can read.

The colors at the top of the rainbow, like red, have longer wavelengths, while the colors at the bottom of the rainbow like blue have shorter wavelengths. And that's why Blu-Ray players, with bluish violet lasers can read smaller pits and lands than DVD players which use a red laser. Since Blu-Ray disks have smaller pits and lands, they can pack in much more data to the same surface area.

Creating a blue laser wasn't easy, however. For one thing, scientists needed a specific form of crystal called gallium nitride, one with extra holes, or lower-energy spots for higher-energy electrons to drop down into, emitting blue light. The physicists who figured out how to do it in the early 1990s eventually won the 2014 Nobel prize in physics, and in 1996 they started using the crystal to make blue lasers.

Over the next decade, blue lasers got better and better until they were reliable enough to be used in a new generation of players that can read disks that hold a lot more information, including Blu-Rays. And that is why your huge movie file fits neatly on one little disk.

Thanks for asking and thanks especially to our patrons on Patreon who keep these answers coming. If you'd like to submit questions to be answered or get these quick questions a few days before everyone else, go to Patreon.com/SciShow, and if you want to keep getting smarter with us, you can go to YouTube.com/SciShow and subscribe.

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