If I am standing on a chair, and I point a laser at a coffee table a few meters away from me (that is also below me), the light should reflect away from me. Therefore, no light should be returning to my eyes right? Why is it that I can still see the spot where the laser pointer is hitting the table. How is the light returning to my eyes when it is reflecting away from me?
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1$\begingroup$ Do you imagine that the surface of the coffee table is perfectly flat? $\endgroup$– TriatticusCommented Jul 7, 2020 at 3:07
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1 Answer
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The coffee table's surface is not very smooth. It has many microscopic bumps and crevices that reflect light in all directions, including back toward your eye. This is called diffuse reflection, as shown in the diagram below.
Light rays only all reflect in the same direction if the surface is smooth on a microscopic level. Mirrors are smooth enough for this "normal" kind of reflection (specular reflection) to happen, shown in the diagram below. So is the surface of a still pond, or a bit of polished metal.
You'll notice that the laser spot on a mirror is much dimmer than on a coffee table, precisely because reflection from a mirror is mostly specular, so most of the light reflects away from you.
answered Jul 7, 2020 at 3:08
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$\begingroup$ The diagram really helped me understand. In theory, a perfectly specular mirror would make it impossible to see the laser, correct? $\endgroup$– JayCommented Jul 7, 2020 at 4:18
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1$\begingroup$ @Jay It would be impossible to see the laser spot on the mirror, yes. But if the laser is passing through a gas, like air, some laser light will scatter off of the gas molecules in its path (Rayleigh scattering), which means you'll see the "beam" (this is fairly obvious with the ultra-bright green laser pointers, but is usually very faint with the more common red laser pointers). $\endgroup$ Commented Jul 7, 2020 at 4:29