Why does a blu-ray disk store more information than a DVD?

CD’s, DVD’s were the most common way to store large amounts of data. Now blu-ray disks have become more prominent, replacing DVD’s as the way to store movies and video games. How do these disks store the information?

Materials

  1. DVD
  2. Blu-ray
  3. Laser pointer
  4. Protractor
  5. Printing paper
  6. Cardboard
  7. Glue/tape

Directions

CD’s, DVD’s were the most common way to store large amounts of data. Now blu-ray disks have become more prominent, replacing DVD’s as the way to store movies and video games. How do these disks store the information? Information is written on the disk in binary format (ones and zeros) in the form of bumps and flat areas. On a DVD each of the bumps are very small (320x400x120 nanometers). If you look carefully at a DVD you can see that there are tiny circular tracks. Each track consists of these bumps and flat areas. The separation between the tracks is 740 nanometers. To reach the tracks, a red laser is used. Blu-ray disks store even more information (5 times a DVD). The separation between tracks on a blu-ray disk is 320 nanometers. In order to read this, you need a blue laser. A blue laser has a shorter wavelength which makes it easier to focus tighter to be able to read the closer spacing accurately. 

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Glue or tape white printing paper on a piece of cardboard. Put the DVD/blu-ray on a flat surface and put the cardboard perpendicular to the disk. Shine a laser from above at a slight angle measure the incident angle $\theta_i$ . Measure the angles $\theta_m$ of the light coming back from the disk. m=0 is the 0-th order diffraction which is simply the reflection. m=1 is the 1-st order diffraction. Measure this angle. The spacing can be calculated from the diffraction equation $d(\sin (\theta_m) - \sin (\theta_i) = m \lambda$ Where $d$ is the distance, and $\theta_m$ and $\theta_i$ are the angles of mode and incidence.

 

Explanation

Diffraction is a process where light bends when it passes around an edge or through a slit. In the case of the disks, the adjacent tracks that are written on the DVD’s and blu-ray disks cause the laser light to bend. The smaller the separation between the tracks, the larger the diffraction angle.