Thin FilmsThin Films

Bright ColorsBright Colors Colors in nature are not only Colors in nature are not only

due to reflected wavelengths.due to reflected wavelengths.• Leaves – spectrumLeaves – spectrum

• Butterfly – interferenceButterfly – interference

A thin film in on the butterfly’s A thin film in on the butterfly’s wings creates the iridescent wings creates the iridescent color.color.

Rebound PhaseRebound Phase Reflection from a fast medium Reflection from a fast medium

causes no phase shift.causes no phase shift.• High index of refraction to low High index of refraction to low

index of refractionindex of refraction

Reflection from a slow medium Reflection from a slow medium causes the phase to invert.causes the phase to invert.• Low index of refraction to high Low index of refraction to high

index of refractionindex of refraction

Boundary EffectBoundary Effect At each boundary there are At each boundary there are

both transmitted and reflected both transmitted and reflected waves.waves.

Like the string there can be a Like the string there can be a phase shift.phase shift.• Invert from a slow mediumInvert from a slow medium

Extra PathExtra Path

A thin film offers two separate boundaries for light.A thin film offers two separate boundaries for light.• Initial boundary from air to film – reflection with phase shiftInitial boundary from air to film – reflection with phase shift• Second boundary from film to air – reflection without shiftSecond boundary from film to air – reflection without shift

Destructive PathDestructive Path For a perpendicular ray a For a perpendicular ray a

reflection from the second reflection from the second boundary has an extra path.boundary has an extra path.• Length Length 22dd

If this is equal to a whole If this is equal to a whole number of wavelengths there number of wavelengths there will be destructive interference.will be destructive interference.• First wave shiftedFirst wave shifted

• Shorter wavelength in filmShorter wavelength in film

md 2 ,2,1,0 m

nmd 02

m

dn20

Constructive TooConstructive Too If the path equals to a half If the path equals to a half

number of wavelengths there number of wavelengths there will be constructive will be constructive interference.interference.• Second wave matches firstSecond wave matches first

Different wavelengths are Different wavelengths are enhanced by different enhanced by different thicknesses.thicknesses.

)(2 21 md ,2,1,0 m

210

2

m

dn

Soap FilmSoap Film

Soap film provides different thicknesses to cause Soap film provides different thicknesses to cause different wavelength to undergo interference.different wavelength to undergo interference.

Newton’s RingsNewton’s Rings

Air or water between glass can also act as a thin film.Air or water between glass can also act as a thin film.• Different index of refractionDifferent index of refraction

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