MOTION MOUNTAIN

what is light? 99
Challenge 105 s
Ref. 58
Page 126
Challenge 104 s
F I G U R E 55
Diffraction lines
can be seen
between the
fingers, if one
looks carefully
enough.
(© Chuck Bueter)
F I G U R E 56 The primary and secondary rainbow, and the supernumerary bows below the primary bow
(© Antonio Martos and Wolfgang Hinz).
patterns that appear when several waves superpose.* The interference patterns depend
on the spacing between the fingers.This experiment therefore allows you to estimate the
wavelength of light, and thus, if you know its speed, its frequency. Can you do this?
Historically, another effect was central in convincing everybody that light was a wave:
supernumerary rainbows, the additional bows below the main or primary rainbow. If
we look carefully at a rainbow, below the main red–yellow–green–blue–violet bow, we
observe weaker, additional green, blue and violet bows. Depending on the intensity of
the rainbow, several of these supernumerary rainbows can be observed.They are due to
interference of light triggered by the water droplets, as Thomas Young showed around
1803.** Indeed, the repetition distance of the supernumerary bows depends on the radius
*Wheredoestheenergygoinaninterference pattern?
**ThomasYoung(b.1773Milverton,d.1829London),readthebibleattwo,spokeLatinatfour;adoctorof
medicine,hebecame aprofessorofphysics.Heintroducedtheconceptofinterferenceintooptics,explain-
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net