A thin acetone film ( ) on glass is illuminated from above by monochromatic light of…

PHYSICS · Subjective · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics

Problem

A thin acetone film (

n=1.25

) on glass is illuminated from above by monochromatic light of wavelength

500\;\text{nm}

in air. Initially the film shows a bright reflected fringe corresponding to order

m=3

. The acetone evaporates so the film thins at a constant rate of

100\;\text{nm}\,\text{s}^{-1}

. After how many seconds will the reflected light first appear dark at the same wavelength?