• Summary
  • Contents
  • Subject index

Via 100 entries, 21st Century Psychology: A Reference Handbook highlights the most important topics, issues, questions, and debates any student obtaining a degree in the field of psychology ought to have mastered for effectiveness in the 21st century. This two-volume reference resource, available both in print and online, provides an authoritative source to serve students’ research needs with more detailed information than encyclopedia entries but without the jargon, detail, or density found in a typical journal article or a research handbook chapter. Students will find chapters contained within these volumes useful as aids toward starting research for papers, presentations, or a senior thesis, assisting in deciding on areas for elective coursework or directions for graduate studies, or orienting themselves toward potential career directions in psychology.


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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