Two transparent polymer films are suspended in air and illuminated with white light…
PHYSICS · Single correct · A Level · Physics · Waves · Waves
Problem
Two transparent polymer films are suspended in air and illuminated with white light at normal incidence. Both films have an index of refraction n=1.50. Film X has thickness tX=75nm, and Film Y has thickness tY=150nm. An observer looks at the light reflected from each film.
Which film will appear brighter in blue light of wavelength 450nm in air, and why?
(a) Film Y, because 2ntY equals mλ for some integer m, giving constructive interference for blue light.
(b) Film X, because its thickness is λ/(4n), a textbook “quarter-wave” thickness that maximizes reflection of that color.
(c) Film X, because 2ntX equals (m+0.5)λ for some integer m, producing constructive interference for blue light.
(d) Film Y, because both reflected rays experience the same phase shift, so constructive interference occurs when 2ntY equals mλ.