# NCERT Physics Class 12 Exemplar Ch 15 Communication Systems Part 4

Q 26 (i) The intensity of a light pulse travelling along a communication channel decreases exponentially with distance x according to the relation where is the intensity at and is the attenuation constant.

Show that the intensity reduces by per cent after a distance of

(ii) Attenuation of a signal can be expressed in decibel (dB) according to the relation. What is the attenuation in dB/km for an optical fibre in which the intensity falls by per cent over a distance of km?

Ans:

where is the attunation in dB/km.

Here

Or

Or

Q 27 A MHz sky wave takes to reach a receiver via re-transmission from a satellite km above earth’s surface. Assuming re-transmission time by satellite negligible, find the distance between source and receiver. If communication between the two was to be done by Line of Sight (LOS) method, what should size and placement of receiving and transmitting antenna be?

Ans: A 50 MHz Sky Wave Above Earth’S Surface A 50 MHz sky wave above earth’s surface