Transfer of energy from a wave to the medium through which it passes. The energy of the wave can be reflected, transmitted, or absorbed. If the medium absorbs only a fraction of the energy, it is said to be transparent to that energy. When all energy is absorbed, the medium is opaque. All substances absorb energy to some extent. For instance, the ocean appears transparent to sunlight near the surface, but becomes opaque with depth. Substances absorb specific types of radiation. Rubber is transparent to infrared radiation and X rays, but opaque to visible light. Green glass is transparent to green light but absorbs red and blue light. Absorption of sound is fundamental to acoustics; a soft material absorbs sound energy as the waves strike it.
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