blue flash

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blue flash (plural blue flashes)

  1. (meteorology, astronomy) A very rare phenomenon observed in the morning or evening when the sun is crossing or immediately below the horizon, in which a momentary flash of blue light appears above the upper rim of the solar disk, caused by refraction of light in the atmosphere.
  2. (nuclear physics) A phenomenon sometimes observed when a criticality incident occurs in nuclear materials, inducing a discharge of radiation into the environment.

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