IB Physics/Digital Technology
14.1 Analogue and digital signals
14.1.1 Solve problems involving the conversion between binary numbers and decimal numbers
In the binary system, only 0 and 1 are used to describe a number. Each digit, from the right end, corresponds to 20, 21, 22, and so on.
e.g. 101001(2) = 1×25 + 0×24 + 1×23 + 0×22 + 0×21 + 1×20 = 41.
The first nonzero digit in a number given in a binary form is called MSB (Most Significant Bit), and the last digit in a number given in a binary form is called LSB (Least Significant Bit).
14.1.2 Describe different means of storage of information in both analogue and digital forms
- Cassette tape
- Floppy disk
- Hard disk
- CD (Compact Disk)
- DVD (Digital Versatile Disk)
14.1.3 Explain how interference of light is used to recover information stored on a CD
The inner working of a CD is composed of pits and bumps - or sometimes called islands and bumps. A laser is used to read these pits and bumps (the rings of the CD). As the laser hits a pit, light reflects off it, returning a signal of 1 (constructive interference). However, when the laser hits the edge of the bump, destructive interference takes place (signal of 0) and the incident and reflected ray have a phase difference of wavelength/2. Therefore, the pit depth/bump height can be approximated to wavelength/4.
14.1.4 Calculate an appropriate depth for a pit from the wavelength of the laser light
pit depth = 1/4*wavelength
14.1.6 Discuss advantage and disadvantage of the storage of information in digital rather than analogue form
- Capacity of data storage is huge
- The access to particular storage data is fast
- The retrieval of data is fast
- The storage is more reliable
- Data can be processed and manipulated by a computer
- Stored data can be copied or erased easily
A serious error with a digital storage is usually catastrophic (data may never be recoverable), while analogue data degrades slowly.
14.1.7 Discuss the implications for society of ever-increasing capability of data storage
14.2 Data capture; digital imaging using charge-coupled devices (CCDs)
14.2.1 Define capacitance
Capacitance (C) is the ability of storing charge. Its unit is F (Farad). Q=CV, where Q is charge flows through the capacitor, and V is voltage.
14.2.2 Describe the structure of a charge-coupled device (CCD)
CCD is a silicon chip with surface area equals to approximately 20x20mm2 to 60x60mm2 covered with light-sensitive elements called pixels (picture elements). Size of each pixel is 5-25x10-6m. Each pixel can be considered to behave as a capacitor.
14.2.3 Explain how incident light causes charge to build up within a pixel
As mentioned before, the pixels behave like a capacitor, which in reality is designed to store charge over a potential difference. When light hits a pixel (of course assuming the light has the minimum frequency), an electron is emitted-the photoelectric effect-and a potential difference is created. Each pixel stores a certain amount of charge and "passes" on the charge in the array of pixels. The intensity of the incident light affects the amount of charge that can be held by each pixel.
14.2.4 Outline how the image on a CCD is digitized
14.2.5 Define quantum efficiency of a pixel
Quantum efficiency is defined as the ratio of the number of photoelectrons emitted to the number of photons incident on the pixel; not every incident photon will lead to emission of photoelectron. Usually quantum efficiency is between 70 to 80%.
14.2.6 Define magnification
Magnification of a CCD is the ratio of the length of the image on the CCD to the length of the object.
Two points on an object will be resolved on a CCD if the images of the points are more than two pixels apart.
14.2.8 Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed image
Quantum efficiency; The greater the QE, the greater the sensitivity of the devices. Magnification; a greater magnification means more pixels are used for a given section of the image. the image will be more detailed. Resolution; The greater the resolution the greater the amount of detail recorded. An improvement in resolution will mean a given image will occupy more memory