Interference...

The physics of thin films and more...

Wave Superposition and Interference• What happens when waves meet?

• Path Difference - Phase Difference

2r

The Physics of Thin Films

• A Phase difference can occur because of…– path differences for different combining

wavetrains– phase shifts introduced under certain

reflections– go to applet thin film

Single, Double and Multi-Slit Interfence• Simple Case: Assume each slit acts as

a single source

• simulate this…– diffraction of water waves– thin slit diffraction

• The Grating Equation:

sinn d

Structural Colour in Plants and Animals

• The iridescent sheen of a bluebird wing or the extravagant colours of many butterflies and moths arise - not from chemistry (pigments) but from physics (interference effects)!!

The Physics of Structural Colour

• Light reflects from slightly different places along the surface. An optical path difference is introduced

Some colours work - some don’t...• The optical path

difference is just the extra distance that one ray travels wrt to an adjacent one. It’s easy to see that this leads to:

for constructive interference

2 / cosl n