Transcript Diffraction and Interference

Diffraction and
Interference
• A wave generated by a single point
source spreads in all directions. After
traveling a large distance, the arriving
wave fronts may appear to be flat.
How did a wave that “flattened out” begin
to spread again after passing through an
opening?
Huygens’ Principle
• Wave Fronts are made up of tinier
wave fronts.
• http://www.launc.tased.edu.au/on
http://www.launc.tased.edu.au/online/sciences/Physics/diffrac.html
line/sciences/Physics/diffrac.html
Huygen’s Principle
• Waves spreading out from a point source
may be regarded as the overlapping of tiny
secondary wavelets, and that every point
on any wave may be regarded as a new
point source of secondary waves.
Huygens’ Principle
• Think: Can two or more small waves
combine into one big wave?
Huygens’ Principle
• Think: Can two or more small waves
combine into one big wave?
Huygens’ Principle
• So can’t you think of one big wave as the
combination of two or more small waves? THAT’S
HUYGENS’ PRINCIPLE!
Diffraction
• A result of Huygens’ principle is that
waves are constantly regenerating
themselves. This becomes apparent
when there is an obstacle or barrier.
Diffraction with water waves
• Notice with the water waves that
the diffraction was more obvious
when the opening was small
compared to the wavelength
• “Kissing Fingers Demo”
Helpful Diffraction
• The longer the wave compared with
the obstruction, the greater the
diffraction.
• This is why AM radio stations get
better reception in the mountains!
They are longer wavelength, so
bend around mountains.
Annoying Diffraction
• Ever wonder why you can’t make a
microscope to “see” an atom in visible
light?
Annoying Diffraction
• Ever wonder why you can’t make a
microscope to “see” an atom in visible light?
• If the size of object is the same as the
wavelength of light, the image will be
blurred by diffraction. Smaller objects won’t
be seen at all
Annoying Diffraction
• Ever wonder why you can’t make a microscope to
“see” an atom in visible light?
• If the size of object is the same as the wavelength
of light, the image will be blurred by diffraction.
Smaller objects won’t be seen at all
• No optical microscope can be built big enough or
designed well enough to overcome this diffraction
limit.
• Electron microscopes used to “see” very small
things
Check your understanding
• Why is blue light used to view tiny
objects in an optical microscope?
Check your understanding
• Why is blue light used to view tiny
objects in an optical microscope?
• Blue light is shorter wavelength, so
diffracts less compared to other colors
of visible light.
Young’s Interference
Experiment
• Is Light a wave or particle? All
waves reflect, refract, diffract, and
interfere.
• We’ve seen everything but
interference so far. Can Light
interfere and cancel out?
Young’s Interference
Experiement
• Used monochromatic light (light of
single color
• Passed the light through two narrow
slits
An Interference pattern!!
Yet another demonstration of light’s wave properties!
Diffraction Grating
• Thousands of tiny slits. Diffracts
light. Colors are produced by
interference between light beams.
• So if you see red, what color is
being cancelled?
•
x = 
•
L
d
• d = distance between slits
• x = distance between central and
1st order
• L = distance from center of slits to
1st order
• Three of these quantities can be
measured in an experiment. What
3?
• X, L and d so with that we can
calculate the 
•
=(xd)/L
• ex
• d=0.02cm x=0.184cm L=80cm
• =(x*d)/L
• = (0.184*0.02)/80 = 4.6*10-5 cm
• Find freq.
• f = c/ = (3*108m/s)/(8.6*105cm*(1m/100cm)) = 6.5*1014
• CD’s and DVD’s are like diffraction
gratings.
Thin-Film Interference
• Have you ever wondered about these?
• Reflections off two surfaces put light
waves out of phase. Some colors are
cancelled. What’s left is what you see.
• In the case of oil and soap bubbles, the
thickness varies throughout, so different
colors cancel in different places