When a light source emits photons, they do indeed propagate in various directions, creating a spherical wavefront that expands outward. However, the photons that are not directed towards our eyes do not contribute to our perception of the light source.
Our ability to see an object or a light source depends on the photons that reach our eyes and interact with the photoreceptor cells in our retinas. These photons enter our eyes through the pupil and pass through the lens, which focuses them onto the retina.
The photons that reach our eyes from a light source follow a straight-line path from the source to our eyes. These are the photons that travel in a specific direction that allows them to enter the pupil and form an image on the retina. They carry information about the location, intensity, and color of the light source.
Photons that do not intersect our eyes or enter the pupil, such as those emitted in other directions, do not interact with our retinas and, therefore, do not contribute to our perception of the light source. While the overall spherical wavefront expands outward, only a fraction of the emitted photons are actually detected by our eyes.
In other words, our eyes act as selective detectors, only capturing the photons that enter our visual system from a particular direction, and those are the ones that allow us to perceive the light source. The rest of the photons, even though they exist in the surrounding space, do not contribute to our vision of the light source because they are not interacting with our eyes.