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Homework Help: Find the number of bits that can be stored on a 10cm disk

  1. Mar 21, 2009 #1
    1. The problem statement, all variables and given/known data

    A computer disk is made by focusing light to a spot whose diameter is comparable to the optical wavelength. Find the number of bits that can be stored on a 10cm diameter optical disk using an infrared laser.

    2. Relevant equations

    Lamda λ = diametre of laser

    3. The attempt at a solution

    I started by investigating the way this works. A disk would be pressed and it will display pits and lands. Each pit width is 500nm and the land witdth would be 1.6micrometers. As the laser reads each land and pit it will interpret each land and pit as a bit 0 and each transition between land and pit as a 1 bit. Since the laser will cover the same length as its wavelength I tried to find how many pits and lands I could fit under that length (this is about 780nm for an infrared light.

    Im thinking Im going about it the wrong way. any ideas?
  2. jcsd
  3. Mar 21, 2009 #2


    User Avatar
    Gold Member

    Hard to say if you're over-thinking it by analyzing the way it works. [Caveat: I don't know how these questions work] but it seems to me there's lots of room for intrerpretation, and thus a range of correct answers. As long as you show your rationale at each step and follow it through to the answer, it seems you'd get a valid, defensible answer.

    At its simplest, it's just a matter of how many 500nm spots you can fit on a disk of that width.
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