#### Transcript Interference

Wave Optics The physics of interference, diffraction, thin films and more... Wave Superposition and Interference • What happens when waves meet? • Path Difference - Phase Difference: illustrate this with the following applet r 2 r Double and Multi-Slit Interfence • Simple Case: Assume each slit acts as a single source • The Grating Equation for Antinodes: n d sin where d = separation between slits Interfence Patterns • Use the following applet to help explore the different interference patterns produced by single, double and multiple slits... Single Slit Diffraction... • A single slit can interfere with itself! simulate this • Equation for Nodes: n W sin Poisson’s Spot - a curious tale! • By the early 19th century the wave model was becoming accepted and the French Physicist Fresnel had worked out an elaborate mathematical theory of light. However, the famous physicist Poisson claimed to have found a fatal flaw in the theory... 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 Path Differences to Phase Shifts... How to calculate phase shift: • • • • Find path difference Divide by wavelength of light IN THE FILM Add any additional phase shifts produced by reflections. If the phase shift is: – 0.5, 1.5, 2.5, …. “n.5” then destructive interference will result – 0,1,2,3,… n then constructive interference will result 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: 2l / cos n for constructive interference Application… Lens Coating • Why do some lens have a purple colour? • Use the applet thin film to help understand the power of the “quarter wave”! So - if it’s a wave - What Kind of Wave? • Known since antiquity - some crystals (calcite) can cause light to refract into two distinct paths when passing through the crystal (1669 - Bartholinus) • Early 19th century, light under glancing collisions can exhibit a “new property” polarization • Light is a transverse wave