Q.What are line communication and space communication? Give examples of each.

Information originates at the source, is sent by the transmitter through a channel, picked up by the receiver, and finally reaches the user.

A basic communication system has five essential elements, arranged in the order the signal actually flows:

1. Information (source) — the message/signal to be communicated (e.g. voice, data).
2. Transmitter — processes the message signal (e.g. modulates it) and sends it into the communication channel.
3. Channel — the physical medium (wire, free space, optical fibre) that carries the signal from transmitter to receiver, and where noise may be added. …

Twisted pair, coaxial cable, and fibre-optic cable are all physical (wired) transmission media; radio waves travel wirelessly through space.

• Twisted pair, coaxial cable, fibre-optic cable: these are all physical conductors/waveguides — the signal is guided along a wire or fibre, so they are wired media.
• Radio wave: this is an electromagnetic wave that propagates through free space (or the atmosphere) without any physical conductor — it is a wireless medium. …

For efficient radiation an antenna must resonate with the signal, which requires its physical size to be comparable to the wavelength; in practice a size of at least about a quarter-wavelength ($\lambda/4$) is the standard rule of thumb (as in a quarter-wave monopole antenna).

An antenna radiates efficiently only when currents/charges in it oscillate coherently over a length comparable to the wavelength of the signal, so that the radiated fields from different parts of the antenna add constructively. If the antenna is much smaller than $\lambda$, the radiated power becomes negligibly small.

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This is from the History/Elements-of-Communication-Systems topic, not part of the current (post-2023 rationalisation) NCERT Class 12 Physics syllabus — answered here for practice, but flagged out-of-syllabus.

Matching each communication milestone to its commonly cited year:

• World Wide Web — 1989-91 (Tim Berners-Lee) → (d)
• Fibre optic communication — 1975 → (c) …

A modem converts a computer's digital data into a modulated signal suitable for transmission and back again, enabling data transfer between computers.

To send data from one computer to another over a communication channel (such as a telephone line), the digital data must be converted (modulated) into a signal that the transmission medium can carry, and converted back (demodulated) into digital data at the receiving end. The device that performs this modulation-cum-de …

Attenuation is the loss of a signal's strength as it propagates through a transmission medium.

In an electronic communication system, as a signal travels through a channel (wire, cable, free space, etc.), it progressively loses power/amplitude due to the resistance and other losses of the medium. This loss of strength during propagation is called attenuation. It is one reason repeaters/amplif …

Fax transmission converts a printed page into an electrical signal through the process of scanning.

In a facsimile (fax) transmitter, the document to be sent is illuminated and a small spot of light is moved systematically across and down the page (a raster scan) while a photodetector/photocell measures the reflected light intensity at each point. Dark (printed) regions reflect less light and light (blank) regions reflect more, so the photodetector's output current varies in step with the pattern on the page — this varying current IS the electrical signal representing the document. This whole line-by-line, point-by-point conversion process is called scanning.

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Broadcast communication sends one signal from one transmitter to many receivers simultaneously (one-to-many), as in radio/TV.

In a communication system, information can be sent either point-to-point (from one specific transmitter to one specific receiver, e.g. a telephone call) or as broadcast. In broadcast communication, a single transmitting station radiates the modulated signal (carrying audio and/or video information) into space, and this signal can be picked up simultaneously by any number of receivers within its range, without the transmitter needing to know who or how many are listening/watching. Classic examples are AM/FM radio broadcasting and television broadcasting, where the same programme reaches every receiver tuned to that frequency within range.

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