Hearing/Seeing Waves: Communicating Without Sight

[a1234]

Imagine that one person is on one side of a hall, and another person is on the other side. There is a gap in the wall that separates the two people. In that case, how is it possible that one person can hear the other person, but cannot see them unless in a direct line with them?

 

[Drakkith]

I think there are 2 key reasons. One is that sound waves easily diffract after passing through the gap. Diffraction is basically the spreading out of a wave after it passes a barrier or boundary. For example, if you're walking down the street you can hear the sound of someone's voice who is around the corner before you pass the corner. Light waves undergo diffraction too, but because their wavelengths are so much smaller they don't spread out nearly as much. This means that you actually have to pass the corner before you can see them.

The 2nd reason is that in order to see someone, you need go through a complicated process to form an image of them on your retina. Light waves are emitted from all over your body and if portions of these light waves overlap when they are focused down on your retina, then the resulting image is blurred. So even if diffraction did occur to a much greater extent, it wouldn't really matter as the light waves from different areas of your friend's body would all be overlapping and you'd just see a big blurry mess.

To hear someone, you just need to have some portion of the sound waves reach your eardrum. Sound waves from someone's voice are all emitted from essentially one place and strike your whole eardrum all at once. So even after diffracting around a corner or after passing through a gap (and then reflecting around through your external ear and ear canal), you can still easily understand what someone's saying.