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Images of objects that do not occupy a single point

The above discussion relates to the formation of an image by a "point object" - in this case, a small light bulb. The same principles apply to objects that occupy more than one point in space. For example, a person occupies a multitude of points in space. As you sight at a person through a lens, light emanates from each individual point on that person in all directions. Some of this light reaches the lens and refracts. All the light that originates from one single point on the object will refract and intersect at one single point on the image. This is true for all points on the object; light from each point intersects to create an image of this point. The result is that a replica or likeness of the object is created as we sight at the object through the lens. This replica or likeness is the image of that object. This is depicted in the diagram below.


Now speaking about image formation, we will turn our attention to the use of ray diagrams to predict the location and characteristics of images formed by converging and diverging lenses.

In electronics, a digital-to-analog converter (DAC, D/A, D2A or D-to-A) is a function that converts digital data (usually binary) into an analog signal (current, voltage, or electric charge). An analog-to-digital converter (ADC) performs the reverse function. Unlike analog signals, digital data can be transmitted, manipulated, and stored without degradation, albeit with more complex equipment. But a DAC is needed to convert the digital signal to analog to drive an earphone or loudspeaker amplifier in order to produce sounds (analog air pressure waves).

DACs and their inverse, ADCs, are part of an enabling technology that has contributed greatly to the digital revolution. To illustrate, consider a typical long-distance telephone call. The caller's voice is converted into an analog electrical signal by a microphone, and then the analog signal is converted to a digital stream by an ADC. The digital stream is then divided into packets where it may be mixed with other digital data, not necessarily audio. The digital packets are then sent to the destination, but each packet may take a completely different route and may not even arrive at the destination in the correct time order. The digital voice data is then extracted from the packets and assembled into a digital data stream. A DAC converts this into an analog electrical signal, which drives an audio amplifier, which in turn drives a loudspeaker, which finally produces sound.

There are several DAC architectures; the suitability of a DAC for a particular application is determined by six main parameters: physical size, power consumption, resolution, speed, accuracy, cost. Due to the complexity and the need for precisely matched components, all but the most special DACs are implemented as integrated circuits (ICs). Digital-to-analog conversion can degrade a signal, so a DAC should be specified that that has insignificant errors in terms of the application.

DACs are commonly used in music players to convert digital data streams into analog audio signals. They are also used in television sets and mobile phones to convert digital video data into analog video signals that connect to the screen drivers to display monochrome or color images. These two applications use DACs at opposite ends of the speed/resolution trade-off. The audio DAC is a low speed high resolution type while the video DAC is a high speed low to medium resolution type. Discrete DACs would typically be extremely high speed low resolution power hungry types, as used in military radar systems. Very high-speed test equipment, especially sampling oscilloscopes, may also use discrete DACS.

3. Read the texts without a dictionary and retell them:

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<== попередня сторінка | наступна сторінка ==>
Diverging lens image formation | Television and telecommunication

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