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Defects of vision and their correction

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The main features of the human eye     This image is a simplified diagram of the human eye.      In many respects it is similar to the camera . An image is formed by the eye lens on the sensitive retina at the back of the eye, whence a message is transmitted to the brain by way of the optic nerve . The iris the colored membranous disk,with a pupil in the center. It has two types of muscular fibers: circular and radial . In the bright light   the circular fibers contract and the radial fibers relax : the pupil becomes less in size to reduce the amount of light that enters. in the less light the circular muscles relax, and the radial ones contract. The pupil then becomes wider so that more light will enter to facilitate vision. in other words, the colored portion of the eye automatically adjusts the size of the pupil or circular opening in its center according to the intensity of the light falling on it. Focusing of the image...
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Attenuators

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Introduction An attenuator is a device for introducing a specified loss between a signal source and a matched load without upsetting the impedance relationship necessary for matching. The loss introduced is constant irrespective of frequency; since reactive elements (L or C) vary with frequency, it follows that ideal attenuators are networks containing pure resistances. A fixed attenuator section is usually known as a ‘pad’. Attenuation is a reduction in the magnitude of a voltage or current due to its transmission over a line or through an attenuator. Any degree of attenuation may be achieved with an attenuator by suitable choice of resistance values but the input and output impedances of the pad must be such that the impedance conditions existing in the circuit into which it is connected are not disturbed. Thus an attenuator must provide the correct input and output impedances as well as providing the required attenuation. Two-port Networks Networks in which electrical energy...
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Transmission Lines

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Introduction A transmission line is a system of conductors connecting one point to another and along which electromagnetic energy can be sent. Thus telephone lines and power distribution lines are typical examples of transmission lines; in electronics, however, the term usually implies a line used for the transmission of radio-frequency (r.f.) energy such as that from a radio transmitter to the antenna. An important feature of a transmission line is that it should guide energy from a source at the sending end to a load at the receiving end without loss by radiation. One form of construction often used consists of two similar conductors mounted close together at a constant separation. The two conductors form the two sides of a balanced circuit and any radiation from one of them is neutralised by that from the other. Such twin-wire lines are used for carrying high r.f. power, for example, at transmitters. The coaxial form of construction is commonly employed for low power use, one co...
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