Chapter 14: Digital technology

Question 1

Explain how interference of light is used to recover information stored on a CD

Answer 1

(Students must know that destructive interference occurs when light is reflected from the edge of a pit.)

Question 2

Capacitance

Answer 2

The size of the potential difference is proportional to the amount of stored charge
C=Q/V

Question 3

Describe the structure of a charge coupled device (CCD).

Answer 3

(Students should know that a CCD is a silicon chip divided into small areas called pixels. Each pixel can be considered to behave as a capacitor.)

Question 4

Explain how incident light causes charge to build up within a pixel.

Answer 4

• The CCD is a small slice of silicon that has been divided into many tiny squares (pixels). Each pixel is essentially a capacitor that stores charge which is released from the silicon when light shines on it. In the case of a CCD pixel, it is a capacitor that uses silicon as its dielectric.
• When silicon absorbs a photon of light, an electron is excited, and becomes free to move within the structure of the silicon, and thus is collected on the capacitor plates. The amount of charge, and hence capacitance of a CCD pixel is therefore directly related to the amount of light falling on that particular CCD.

Question 5

Outline how the image on a CCD is digitized

Answer 5

(Students are only required to know that an electrode measures the potential difference developed across each pixel and this is then converted into a digital signal. The pixel position is also stored.)

Question 6

Quantum efficiency

Answer 6

the ratio of the number of photoelectrons emitted to the number of photons incident on the pixel.

Question 7

Magnification

Answer 7

the ratio of the length of the image on the CCD to the length of the object.

Question 8

Describe a range of practical uses of a CCD, and list some advantages compared with the use of film.

Answer 8

oEach photo does not require the use of film and is thus cheaper
oThe image is digital so it can be enhanced and edited using electronic processing techniques
oImage can be viewed virtually immediately – there is no processing time
oStorage and archiving of a large number of photographs is easy

Question 9

Outline how the image stored in a CCD is retrieved.

Answer 9

The information encoded in digital format is used to reconstruct the image as the information…