Answer:
Reflection, absorption and re-emission.
To clarify, you mentioned light is not visible. Of course that is not true, we can see light. our retinas are sensitive to light within a certain range of frequencies. you may be commenting on the fact that if there is a beam of light in front of me traveling from right to left, I won’t see it. That’s true. If it were a ball I would see it. That is because light strikes the ball, is reflected, absorbed, and re-emitted or some combination of the above, then that light strikes my eye.
As a side note, if the ball were somehow changed by the light which struck it (change of course due to momentum of light impact) then I may not be able to trust that the image I see truly represents the current condition of the ball. But for most balls we can see, this is not the case. If, however, we do go smaller in our attempt to ‘see’ light, we won’t necessarily be able to trust our instinctive intuition. That is informed by the large scale world we observe with our unaided eye. We would have to learn to trust our observations however those observations come. And the observed truth of the very small, the world of quantum physics, is rather different than our unaided perception. It is nevertheless equally valid.
Back to the main point.
If you like to play the semantics game of asking whether you see the ball or see the light, suit yourself. The ball interacts with light, the light interacts with my eye, my eye sends data to my brain for interpretation. I’m fine with either version of ‘seeing’ the ball or ‘seeing’ the light.
Here’s an example to hopefully clarify.
I’m outside on a sunny day, no electric lights around, only sunlight. We all know sunlight is well collimated, one-directional. I’m in my back yard in the shade of my awning. No direct sunlight strikes me or the ball in my hand.
How do I see the ball in my hand?
Direct sunlight strikes a number of objects around me, the ground, my pool, etc. I don’t see any of that light. All of those objects reflect some light. That reflected light leaves the objects at certain limited angles based on the angle of incidence of the sunlight striking that surface. They also absorb some light. They then re-emit the light they absorb. Based on the surface characteristics, this re-emitted light shoots out in all directions away from the surface. They all become sources of multi-directional light nearby, just much lower intensity. Like a bunch of little light bulbs. So the ball I hold (in the awning’s shadow) is lit from a number of sources, just much more dimly than the ground lit by direct sunlight. And so the ball appears darker. It is in the shadow after all.
By the same principle, if the part of Earth you are standing on is in the shadow of the rest of the Earth, i.e. it is nighttime, then you won’t see anything. Unless of course there are other objects around Earth (moon, et al.) that absorb and re-emit light. When the sun, Earth, and moon are lined up so that the sunny side of the moon has a lot of its surface area facing towards Earth (full moon) then there is a lot of reflected and re-emited light striking Earth. Those are nights when it is easier to see by moonlight, which is of course sunlight.
