Eli5 Why light waves require line of sight while sound waves do not?
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They don’t really. Think of someone in the next room turning on a light and you’re around a corner. Those light waves are reflecting off of walls and other surfaces and you see them. If you look outside your window during the day, you see what is outside. That is reflected sunlight. You don’t need direct line of sight because light will reflect off of objects. That is literally how you see.
Sound does the same thing. You’ll hear it best directly from the source, but you’ll still hear it as sound waves bounce off of the environment.
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The clue is in the question: line of *sight*. That’s literally the definition of what’s required for visible light to travel.
Because they are something completely different.
Light is electromagnetic radiation… (Visible light is only small part of the spectrum)
Sound is vibration. This means it can vibrate other things. It can travel through solid matter and back to air.
Both light and sound can bounce or reflect. Think about being in a room where you can’t see a window and there is no light on, but it’s light outside. You can still see in the room, right? That’s because the light from the window is bounding off of the walls. Sound does the same thing.
You can be in an environment where neither light nor sound will bounce. In an anechoic chamber (a specialized room / chamber that is designed to prevent reflections of sound), you would need direct line of sight (sound?) to hear a sound. If the chamber was also perfectly black in color (or otherwise did not reflect light), you would also need line of sight to see light.
The reason why you can’t “see” around a corner with the same resolution as you can hear is because with sound, you don’t care about the angle it hits you from. You lose some details, but the pitch of the sound doesn’t change too much even if it bounces around a bunch.
You might be able to see the color of a tinted lamp around a corner (as the color, like sound, is defined by frequency) but the exact image of the lamp is lost as the light hits a wall and scatters randomly in different directions.
If the light reflected evenly to allow you to see around a corner, then what you have is a mirror.
Sound travels through a medium. Its pushing molecules around. Those molecules push other molecules, which push other molecules…
You get the idea.
Since solid surfaces tend to push back, you can get sound bouncing off to other directions.
Sound also usually radiates from a source, like a drop of water in a pond. Actually thats a great way to see how vibrations travel. It will create smaller waves when it hits some object, and propagate from there.
Light (and electromagnetic energy) is… different. For our purpose, we say it acts like a particle most of the time, for everyday experience. It doesn’t (that we know of) push other _somethings_ around. A photon is created, travels in one direction at the speed of light. It may then absorbed into an electron and then reemitted at a different wavelengh (which is how we get colors).
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.
The key is wavelength.
Visible light wavelength is in the 0.00000001 m. This is very short.
Sound wavelengths are much longer. Normal speech is around 1-2 meter wavelength.
When a wave hits an corner it diffracts. This happens with waves of all types: light, sound, waves in water etc. It looks omething like this: https://web2.ph.utexas.edu/~coker2/index.files/waveobstacle-diffraction.gif
The amount of diffraction depends on the wavelength.
Short wavelength waves (like light) diffract very little.
Long wavelength waves (like sound) diffract very strongly.
So light that comes to a corner mostly keeps on going straight. But sound that hits corner will bend behind the corner.
So you can hear the sound coming from behind the corner but you can’t see light coming from behind the corner.
Sound waves must be carried on a medium such as air or solid matter.
Light waves do not require a medium. Light can travel through the vacuum of space.
Light is also affected by gravity. Massive objects bend space. In other words, gravity defines the meaning of “line of sight.”
During a solar eclipse, astronomers can see stars, that from Earth’s perspective, are behind the Sun.
Astronomers can see galaxies behind closer groups of galaxies. The further galaxies appear distorted as [Einstein Arcs and Rings.](https://en.wikipedia.org/wiki/Einstein_ring) (See the gallery in the article.)
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