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The excitation band of a typical stilbene fluorescent whitener lies near

  • A.550 nanometres in the green region where the eye is most sensitive
  • B.650 nanometres in the red region at the long wave end of the range
  • C.350 nanometres, just short of the violet end of vision
  • D.435 nanometres in the blue part of the visible spectrum itself

Answer: C

Excitation occurs in the near ultraviolet around 340 to 370 nm. Emission is in the visible blue near 435 nm. The Stokes shift between the two bands is what allows invisible radiation to be converted into visible light and so raise the apparent whiteness.

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