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=64
30368900309572093#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: