Question

Why can’t you see the beam of light itself though you can see the bright spot on the wall or on a piece of paper that intersects the beam? Explain your reasoning.

Answer 1

From basic law of optics, we know that we can only see something when it reflects off the incident light on it. And the brightness depends upon the intensity of light.

Now, we see the bright spot on a paper because it illuminates the paper as it gets reflected off of the paper. But , in order for the beam itself to be seen, there must be something in the air itself to reflect it.

Now, here comes the tricky part. The air does contain gas molecules and dust particles. But thode way too small and scattered to reflect a considerable amount of light back.

This is the main reason we don't see the beam. Although we do see dust particles floating in air around a dusty room or a place where there is some consideration amount of large dust particles.