beam splitter

Definition: Optical component e.g. prism, system of mirrors or semi-reflecting mirror (pellicle) used to divide up a ray or bundle of light. (1) TRANSVERSE type: beam is divided into different parts across its axis, each part carrying different part of the image e.g. multi-image prism or lens attachment for stereo photography. (2) TOTAL type: light energy across whole beam is divided up for use, each part carrying same image but at different power e.g. secondary mirror in auto-focus SLR cameras, pellicle mirror in SLR cameras or when creating reference and object beams in holography. * Also used to combine two or more beams of light e.g. in front projection systems: semi-reflecting mirror allows combination of image of projected slide with main subject.
More splitting than you can shake a glass at
Strictly speaking, light is always split on entry into a medium and on exit from it: at the change of refractive index, part of the beam is refracted while part is reflected back.

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