The phenomenon you're referring to, where we can only see the point source or final destination of light and not its transit, is known as the "point source localization" or "ray optics approximation." It is based on the principles of geometric optics, which provide a simplified description of how light propagates.
In geometric optics, light is treated as a collection of rays that travel in straight lines. These rays are used to represent the path of light from a source to an observer or from an object to our eyes. According to this approximation, the rays of light do not interact with each other and do not spread out during propagation. Instead, they travel independently in straight lines until they reach a surface or are absorbed by an object.
When we observe a point source of light, such as a distant star or a small light bulb, we see the light that reaches our eyes directly from that source. The rays of light from the source travel in straight lines and enter our eyes, allowing us to perceive the source as a point of light.
Similarly, when we observe the final destination of light, such as a screen or a surface where light is absorbed, we again see the light that reaches our eyes directly from that surface. The rays of light from the surface enter our eyes, enabling us to perceive the final position of the light.
However, during the transit of light between the source and the destination, the individual rays of light are not directly observed. Instead, they propagate through space and can interact with other objects or surfaces along the way. In the context of the double-slit experiment, where light passes through two closely spaced slits and creates an interference pattern on a screen, the transit of light is not directly visible to our eyes.
The reason for this limitation lies in the nature of light itself and the scale of observation. Light behaves as both a wave and a particle, and its behavior at the microscopic level is described by quantum theory. In the case of the double-slit experiment, the wave nature of light leads to interference effects, creating a pattern of light and dark regions on the screen. However, the interference pattern is a result of the collective behavior of many individual photons, and it is not possible to observe the transit of each individual photon through the slits.
In conclusion, the inability to observe the transit of light between a source and a destination is a consequence of the simplifying assumptions of geometric optics, which treat light as independent rays traveling in straight lines. This approximation is valid for many everyday situations but fails to capture the full complexity of light's behavior, particularly in quantum phenomena such as the double-slit experiment. To understand the complete picture and explain such phenomena, quantum theory is necessary.