W9_parallel_resonance.eps

452 Week 13: Interference and Diffraction

13.11: Thin Film Interference

d n

1

2

n 3

Incident Light

n

Reflected Light

π

π

n 1 <n 2 <n 3

(Two phase shifts of π)

Figure 192: One of the two basic diagrams for thin film interference.The total phase difference in
the superposed reflected waves in the casen 1 < n 2 < n 3 orn 3 < n 2 < n 1 is justδ=k′(2d), as the
phase shifts produced by reflecting off of the two surfaces are either both zero or both (as they are
in this case)π, in which case they cancel.

d n

1

2

n 3

Incident Light

n

Reflected Light

π

n 1 <n 2 >n 3

(One phase shift of π)

Figure 193: The second of the two basic diagrams for thin film interference. The total phase difference
in the superposed reflected waves in the casen 1 < n 2 > n 3 orn 3 < n 2 > n 1 isδ=k′(2d) +π, as
there is a phase shift ofπproduced by reflecting off of the surface of a material with a higherindex
of refraction only one one of the two surfaces..

Observing interference from slits thick or thin, at optical frequencies, is a bit of a rarity in

everyday life. We just don’t trip over visible light travelling through multiple pathways

within the coherence length of the lightto reach a common goal every day, given that the

coherence length of light from hot/chaotic sources is the order ofa few microns (tens to

perhaps a hundred wavelengths). Exceptions do include – for a fewpeople – diffraction

limited viewing through visible light telescopes and microscopes, discussed above, or

people who use spectrographs based on diffraction gratings. Well, Isuppose I should

include the rainbow of colors one can see on the bottom of CDs or DVDs, which are

basically reflection-based diffraction gratings as light bounces off ofthe many tiny tracks

scored in the reflective surfaces –nowthat is an everyday experience but it hasn’t always

been so.

Thin film interference, however, is something that wemight wellobserve every day, or

nearly so. Every time we blow a soap bubble, or see a slick of oil or gasoline on water,

swirling around with many colors, we are observing thin film interference. Whenever

we look at the lens of a camera and see a lack of reflections or those same “metallic”