有關電磁波的應用的問題

Q: 頻率不同的無線電波在傳遞訊息上，有何差異？

Transmission of radio waves over long distance (transcontinental communication) by reflection at the ionosphere was used in those days before the space era when satellites were still not in existence. Because of the low frequency (~20 MHz) of the radio waves used at that time, only voice signals could be transmitted over long distances, but not video signals.

In fact, the use of high frequency radio waves enable a wide bandwidth that could accommodate more "complex signals" to be carried by the wave. This is the reason why the old GSM mobile phone network (of frequency ~800 MHz) can only be used to transmit voice call. The 3G network, which uses frequency of 2.1 GHz, can be used to transmit both voice and video signals.

Q: 換句話，為何人們要用不同頻率的電磁波傳送資訊？

Nowadays, simple voice signals can still be used by medium frequency range radio waves. Commercial radio transmission, which uses frequencies around 1 MHz (AM radio transmission) or 100 MHZ (FM transmission). Commercial TV signals, which require transmission of both voice and picture, are around 300~600 MHZ.

Q: 微波、紅外線、可見光、X射線和伽傌射線就不能被用作發送資訊的用途嗎？

Ionizing radiation, such as X and gamma rays, is clearly not used in telecommunication because of its harmful effect to human.

Microwave is actually used in telecommunication, such as transmission over extremely long distance by satellites.

Infrared, visible and ultraviolet are not used because of absorption of the wave by the atmosphere and, most importantly, there is already high intensity of these waves in the natural environment which casues great distrubance to the transmitted waves. However, communication using optical fibre is possible, as the transmitted optical signals is confined within the optical fibre, which prevents signal disturbance by other waves in the environment.