When a laser beam travels through the air or a vacuum, it is generally not visible to the human eye because it emits light in a narrow and focused beam of a single wavelength. The beam of light appears invisible because the photons in the laser beam are not scattered or diffused significantly by the particles in the air. As a result, there are no interactions with the particles to redirect or disperse the light in different directions, which would make it visible.
However, when the laser beam interacts with an object or surface, such as a wall or an object placed in its path, several things can happen. First, if the object or surface is capable of reflecting light, the laser beam can be reflected off the surface, and the reflected light can then enter our eyes, making the laser beam visible. This is similar to how you can see any other object that reflects light.
Secondly, if the object or surface is capable of scattering light, it can cause the laser beam to be scattered in different directions. The scattered light can then enter our eyes, making the laser beam visible. This is why you might see a laser beam if you place an object, like smoke or dust, in the path of the beam. The particles in the object can scatter the laser light, making it visible as it interacts with the particles.
In summary, the reason you can't see the laser light when it travels through the air is because it doesn't scatter significantly or interact with particles in the air. However, when the laser beam interacts with objects or surfaces that can reflect or scatter light, it becomes visible to our eyes.