Eli5 Why light waves require line of sight while sound waves do not?
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Generally speaking, waves with low frequencies bounce off most hard surfaces really well. Waves with higher frequency get absorbed more and reflected less. That’s why you can get AM/FM radio fairly deep inside a house, but satellite TV dishes need line of sight to a satellite.
Sound is low frequency. The spectrum audible to humans is 20 Hz to 20kHz. And you can hear loud noises through the wall because the sound waves are also making the wall vibrate at those frequencies, though the material does absorb some of the sound.
AM Radio is kHz; FM is MHz; satellite TV is very high MHz; WiFi is GHz. As the frequencies increase, the waves reflect less and get absorbed more. (WiFi can penetrate wood walls, but only over very short ranges compared to radio or satellite, and does very poorly with concrete walls.)
The light spectrum visible to humans is 400–790 **terahertz**. When light hits most surfaces, a lot of that light is absorbed, with only a small bit reflected away. That’s how we perceive color; a red apple only reflects red and absorbs all other wavelengths; a yellow banana only reflects yellow and absorbs all other wavelengths; etc. And unless the object is highly reflective, the reflected amount of light is a small fraction of the light that was shined on the object, and it’s also being scattered in many directions.
So if you have all lights off and the TV on, you’ll see some light around the corner, but not in a way that lets you tell what’s on the TV. The walls and objects around the house are absorbing most of the light, and only reflecting back some wavelengths, and then only a small amount, and also scattering it in the process. Highly polished (and thus somewhat reflective) surfaces such as smooth wood furniture or a vase might give you a fuzzy picture, but if you want to see the TV screen around the corner, you need something that reflects most of the light without absorbing and scattering it, e.g. a mirror.
And if you’re in a separate room with a closed door, the light from the TV isn’t getting to you. It’s hitting other surfaces—walls, sofa, carpet, door—and mostly getting absorbed. With a bit of light getting reflected back to those with line of sight of those reflections.
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