How come light is invisible but we can see it on a surface?

Question

How come light is invisible but we can see it on a surface?

Answer 1

We see light only when it stimulates the rods and cones of  our retinae, either directly from the source, as when we look at a fire, a glowing lightbulb or a star... or indirectly .. when, as you mention,  it comes to us from the source, after being bounced, or reflected off some surface.

When we look at a headlight beam shining across our line of sight through fog, we are actually just seeing a composite of thousands of water droplets reflecting light at our eyes from the halogen filament of the headlight.   When the beam shines  through transparent air.. or better still, a vacuum, you are right:  it is invisible to us, unless our direct or peripheral gaze points straight at the source so that some of its light strikes our retina.

Answer 2

We actually don't see the surface of the object we are looking at, but rather we see the reflection of light off of it.   So everything is invisible, including light, until light rays touch an object.

Answer 3

Light is not invisible.  If your eyes could not see light, you would be blind or it would be dark all the time.  You can certainly see light coming from the sun and from light bulbs.  The light we see on surfaces is the visible light that hits the surface and interacts with the atoms that make up the surface.  The electronic structure of the atoms and molecules that make up the surface determines which wavelengths are absorbed and which are reflected and, therefore, the color that you eye sees.