Interactions can occur when waves pass from one medium to another, bounce back like an echo, bend or extend around or across edges, or come into direct contact with each other. These ways in which waves can interact with matter are called reflection, refraction, diffraction, and interference. Sound is what happens when vibrations of energy are transmitted through that matter. Sound is composed of wave-like vibrations.
When you speak out loud, the movement of your vocal cords causes the air to vibrate. Air is made up of molecules, and when those air molecules vibrate, they collide with each other. Because of these collisions between air molecules, a sound will spread from one side of the room to the other very quickly. Diffraction is the curvature and propagation of waves around an obstacle.
It is most pronounced when a light wave hits an object with a size comparable to its own wavelength. An instrument called a spectrometer uses diffraction to separate light into a range of wavelengths (a spectrum). In the case of visible light, the separation of the wavelengths by diffraction results in a rainbow. You wouldn't want to create a highly absorbent space in a concert hall, for example, because without a full studio to mix layers of sound, it would sound strange and unnatural.
Bats use this to their advantage and can see in the dark by feeling how the sound waves they produce are reflected off the walls of the cave. Part of the wave will try to pass through solid, liquid, or gas, but most of the wave will bounce back and travel in a different direction. The color of an object is actually the wavelengths of the reflected light while all other wavelengths are absorbed. Therefore, sound moves through some matter in the form of vibrations, but a sound wave can also collide and interact with other matter.
Because of this, the wave is reflected (or bounced) and then moves in a different direction than the one it was originally traveling in. It's just your voice, the sound waves you've produced, that bounce (or are reflected) off the walls and back to your ear. When a light wave encounters an object, it is transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light. A spectrometer uses the diffraction (and subsequent interference) of light from slits or gratings to separate the wavelengths.
Waves are reflected when the density of matter is too high for the wave to pass through it or be absorbed. The opposite of absorption, called sound reflection, can be a good thing because it can make a sound appear natural or rich. The amount of scattering that occurs depends on the wavelength of the light and the size and structure of the object. When a wave is absorbed, matter absorbs energy from the wave and, in so doing, reduces the amplitude.
Many music rooms and concert halls are designed to allow sound to bounce off walls and fill the room. For example, when a sound wave hits a foam padding, energy travels through the material and is sometimes converted into heat or other forms of energy.
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