Today 1/29 Lab Questions? Air wedges, soap films, coatings HW: 1/29 “Air Wedge” handout

Due Friday 1/31 Thursday: Begin Ch 26 Refraction Peer guidance center (211 Wit) staffed:

Mon - Thur, 1-4 Except Mon 2-3 and Tue 3-4You are welcome to use room 211 anytime.

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin FilmsNo phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Thin Films

Trough in Trough out

Crest in Crest out

Incoming wave

Reflected wave

Now invert the reflected wave (phase shift). This is like light bouncing off of a higher n material.

No phase shift means that a trough bounces off a boundary as a trough and a crest bounces off a boundary as a crest.

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Now invert the reflected wave (phase shift). This is like light bouncing off of a higher n material.

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Thin FilmsPhase shift means that a trough bounces off a boundary as a crest and a crest bounces off a boundary as a trough.

Trough in Crest out

Crest in Trough out

Incoming wave

Reflected wave

Antireflection coatings

Setting up destructive interference for reflected light means that it all (mostly) is transmitted.

LensCoating

Can’t do it for all wavelengths at once so choose green, in the middle of the visible spectrum.

When n coating < n glass......both reflected rays phase shifted so 2t = (m + 1/2) coating gives destructive interference in the reflection.

When n coating > n glass...

…only one reflected ray is phase shifted so 2t = (m) coating gives destructive interference in the reflection.

Thin Films

Eyeball

The wavelength is different in the film.

Wavelength ()Film thickness (PLD)Index of refraction (n)Phase shifts

film = vacuum /nfilm

Thin Film Problems

Draw picture Consider reflected wave phase shifts

none, one, or both can be shifted

Find in the film Chose m or (m + 1/2) (use in the film)

Constructive or destructive? Phase shifts?

PLD = 2t for thin films

Example:

665 nm light shines on a soap film nearly perpendicularly. The soap film has n = 1.33. What is the minimum thickness of the film if this wavelength is strongly reflected? For air n = 1.

One is phase shifted (front surface), the back surface reflection is not phase shifted. film = /n film = 665 nm /1.33 = 500 nm

Want constructive but one is phase shifted so...

we use (m + 1/2) film

2t = (m + 1/2) film minimum t when m = 0

2tmin = (1/2) film

tmin = (1/4) film = 125 nm

Example: See example 11 Ch 27

Light shines on a soap film nearly perpendicularly. The film is 375 nm thick and has n = 1.33. List the first four wavelengths of light for which constructive interference causes the film to look bright in reflected light.

2t = ?(m + 1/2) film since only one is phase shifted

)/m(

t2

21film

film air = nfilm film

Visible light

1500 nm500 nm300 nm214 nm

2000 nm665 nm399 nm285 nm

Thin Films, Wedges

Vertical soap film is thicker at the bottom.

Pair of thick glass plates with a thin air wedge in between. The air is the thin film.

Both will produce light and dark fringes because the thickness varies.

Thin Films, Wedges

Soap film in reflected light. Thin at the top.

2t = 1/2 film

2t = 3/2 film

2t = 5/2 film

2t = 7/2 film

2t = 9/2 film

2t < 1/2 film t < 1/4 film!!!

t = 1/4 film

t = 3/4 film

t = 5/4 film

t = 7/4 film

t = 9/4 film

2t = (m + 1/2) film for constructive since only one ray is phase shifted.

Thin Films, Wedges

Air wedge in reflected light. Thin at the left.

One is phase shifted and one is not so for constructive interference....

2t = (m + 1/2) air It’s an “air” film

Top view

Where the glass touches, it will be dark. Why?

Thin Films, Wedges

What about the transmitted light?

No light reflected means all transmitted!

Reflected

TransmittedThis is hard to observe

This is easy to observe