A laser beam effectively "ends" when the light is no longer strongly reflected back, causing it to diminish in intensity to the point of being unnoticeable.
After the beam exits the Powered Beam Line (PBL) - or any laser source - and enters air with fewer aerosols or reflective particles, significantly less light is scattered back towards the observer. This reduced backscatter results in the beam appearing to disappear. Here's a more detailed breakdown:
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Initial Intensity: The laser beam starts with a specific intensity determined by the laser's power and focusing optics.
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Interaction with the Medium: As the laser beam travels, it interacts with the medium it is passing through (typically air). This interaction involves:
- Absorption: Some of the laser light is absorbed by the molecules in the air.
- Scattering: Light is scattered in various directions by particles like dust, water droplets, and aerosols. This scattering is what makes the beam visible from the side. The greater the concentration of these particles, the more visible the beam.
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Beam Divergence: All laser beams naturally diverge (spread out) as they travel. This divergence reduces the power density (power per unit area) of the beam, making it less intense over distance.
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Reduced Backscatter: When the laser beam enters an area with significantly fewer scattering particles (cleaner air), the amount of light scattered back towards an observer decreases drastically.
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Perception: The human eye can only detect light within a certain range of intensities. As the laser beam travels further, the combined effects of absorption, scattering, and divergence eventually reduce the intensity of the remaining light to below the detection threshold of the eye. At this point, the beam appears to "end" or disappear.
In summary, a laser beam doesn't abruptly stop existing; instead, it gradually weakens due to interactions with the environment until it becomes too faint to see. The perceived "end" is where the light intensity falls below the threshold of visibility.
