Laser beams appear to vanish abruptly in the night sky due to a phenomenon called atmospheric scattering. When a laser beam travels through the atmosphere, it interacts with the tiny particles present in the air, such as dust, water droplets, and aerosols.
These particles cause scattering of the laser light in different directions. The scattered light spreads out and weakens as it travels through the atmosphere. As a result, the intensity of the laser beam diminishes with distance. This effect is known as beam divergence.
In the day time, the scattered light from the laser beam is often less noticeable because of the ambient sunlight, which provides a background illumination. However, in the darkness of the night sky, the scattered light becomes more apparent, especially if there are particles in the air that can reflect or scatter the laser light.
As the laser beam travels farther away from its source, the scattered light becomes weaker and eventually falls below the threshold of human perception. This causes the laser beam to appear to vanish abruptly in the night sky, as the remaining light is no longer visible to the human eye.
It's worth noting that some atmospheric conditions, such as fog or haze, can enhance the scattering effect and make the laser beam more visible over longer distances. However, under normal atmospheric conditions, the beam divergence and the scattering of light cause the laser beam to gradually fade and disappear from view.