In an optical communication system, the laser beam transmitting information has the form of a cone with an angle at the vertex of 10−4 rad (divergence angle). In the receiving device, light energy is focused on the photocell using a 1 m lens. It turned out that when the distance between the transmitter and receiver changed from 5 to 10 km, the signal received from the photocell decreased by half (due to light absorption in the atmosphere). How many times will the signal change as the distance increases from 10 to 20 km?
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13.5 The quantum nature of light