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The physical properties of light make it an ideal source of information about the external world. Light is a form of energy emitted by the sun and artificial sources, such as lamps, and travels virtually instantaneously in straight lines, or rays. When light rays encounter an obstacle they may be transmitted, absorbed, or reflected. All three behaviors are important for vision:

  • During transmission through translucent objects, the rays may change direction (refraction), a crucial property of lenses.
  • Absorption by photoreceptors in the eye is necessary for the process of vision to begin.
  • Reflection from an opaque surface provides visual information about its properties—its material substrate, shape, and position.

Paradoxically, light behaves as a particle as well as a ray. Light particles (photons) have a vibration ...

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