Huygen’s Principle, Diffraction and Interference

Huygen’s Principle

• Recall from last unit that Huygen’s was the first person to prove that light must be a wave.

• He did this by demonstrating that light diffracts, or spreads out when it passes a barrier, and that diffraction is solely a wave property.

Huygen’s Principle

• Why do waves diffract? Huygen’s came up with a model of wavefronts that helps explain this.

• Huygen’s Principle: Every wave front is made up of smaller circular wavefronts, called wavelets.

• Remember that waves themselves are vibrations and the source of waves are vibrations. So waves can create other waves.

Diffraction

• Diffraction is the bending of waves as they spread out after passing a barrier. ALL waves diffract.

• Water Waves Diffract: Wave Diffraction Video• Light Waves Diffract: Virtual Hole in Wall• Sound Waves Diffract-This is why we can hear

someone out in the hall. Because the sound waves diffract and spread out as they pass through the door.

Diffraction Explained

• As wavefronts encounter a barrier, some parts of the wavefronts get blocked. However, the wavefronts that pass the barrier will become the source vibrations for new waves that will spread out behind the barrier.

Interference Revisited

• Which one of these is CONSTRUCTIVE INTERFERENCE and which is DESCTRUCTIVE INTERFERENCE?

• CONSTRUCTIVE DESTRUCTIVE

Young’s Double Slit Experiment• Light from a single mono-color source is passed through two slits.

• It works because certain wavelengths will create a pattern of light and dark areas when projected on a screen through two slits that are a known distance apart.

• By measuring the distance between the slits and the distance between the bright spots in the interference pattern, we can calculate the wavelength of the light.

More on Young’s Double Slits and the Wave/Particle Duality of Matter

• Remember how we learned that photons are weird and sometimes behave like waves and sometimes like particles?

• Yeah, well it turns out that matter does the same thing, which is even weirder, if you ask me.

• http://video.google.com/videoplay?docid=6430368900309572093#docid=-4237751840526284618

Thin Film Interference

• When we look at a soap bubble or an oil slick on top of water, we often see different swirling colors

• This is caused by thin film interference, which is caused by reflection off the top and bottom surfaces of the thin film: