Transcript Diffraction and Interference Diffraction Huygen’s Principle Diffraction Lab Light Has wave properties. Can diffract. Can constructively or destructively interfere.

Diffraction and Interference
Diffraction
Huygen’s Principle
Diffraction Lab
Light
Has wave properties.
Can diffract.
Can constructively or destructively
interfere.
Wave Fronts
• Lines that are
perpendicular to
the motion of the
wave.
• Indicate the
location of the
crests in the waves
that are traveling
together.
Huygen’s Principle
• Wave fronts are made up of tinier wave
fronts.
• Every point on any wave front is a new
source for a secondary wave front.
Huygen’s Principle
• You can explain reflection and refraction
using Huygen’s Principle.
http://www.microscopy.fsu.edu/primer/java/reflection/huygens/
Huygen’s Principle
As the straight waves passed through a
narrow hole, they spread out in a
circular pattern.
Giving proof to the fact that every point
on a wave front is a new source for a
new set of wavelets.
http://www.ngsir.netfirms.com/englishhtm/Diffraction.htm
Diffraction
Any bending of a wave around an
obstacle or edges of an opening by
means other than reflection or
refraction.
Diffraction
• The amount of diffraction (bending)
depends on the size of the wavelength
compared with the size of the obstruction.
• The longer the wavelength is compared to
the obstruction, the greater the diffraction.
Demo
Is Diffraction a Good Thing?
Why would we ever want waves to bend
past an obstruction?
Is Diffraction a Good Thing?
Long AM radio waves can diffract around
hills and buildings and can be received
better in more places than short waves
that don’t diffract as much.
Is Diffraction a Good Thing?
Diffraction is bad when
we want to see very
small objects with
microscopes.
If the size of the small
object is the same as
the wavelength of light,
the image will be
blurred by diffraction.
Interference
Young’s Interference Experiment
1801, Thomas Young
discovered that when
light of a single color
(monochromatic) was
directed through two
closely spaced
pinholes, fringes of
brightness and
darkness were
produced on a screen.
Young’s Interference Experiment
• Bright fringes = constructive interference
– Waves arrive at the screen in phase
• Dark fringes = destructive interference
– Waves arrive at the screen out of phase
Diffraction Grating
• A series of closely spaced parallel slits or
grooves that are used to separate colors
of light by interference.
• Different colors have different wavelengths
and diffract at different rates.
• So they constructively interfere at different
places.
Single-Color Interference
from Thin Films
• Interference fringes can be produced by the
reflection of light from two surfaces that are
very close together.
• If you shine a single-color (monochromatic)
light onto stacked (with an air wedge) plates
of glass, you’ll see dark and bright bands.
Single-Color Interference from
Thin Films
• The reason for the dark/bright bands is
that reflected light from the top plate
interferes destructively/constructively
with light reflected from the bottom
plate.
Single-Color Interference
from Thin Films
• Practical uses would be to test the
precision of lenses.
• Straight/round fringes = perfectly
flat/round glass
• Irregular fringes = irregular surface
Iridescence from Thin Films
Iridescence from Thin Films
Iridescence: The phenomenon whereby
interference of light waves of mixed
frequencies reflected from the top and
bottom of thin films produces a
spectrum of colors.
Iridescence From Thin Films
• A thin film, such as a soap bubble or oil on
water, has two closely spaced surfaces.
• Light that reflects from one surface may
cancel light of a certain frequency that
reflects from the other surface.
http://webphysics.davidson.edu/physlet_resources/bu_semester
2/c26_thinfilm.html
Iridescence From Thin Films
• If the film is illuminated with white light
and the light that reflects to your eye
has blue cancelled due to the reflected
light from the other surface, what color
will you see?
Iridescence From Thin Films
• If the film is illuminated with white light
and the light that reflects to your eye
has blue cancelled due to the reflected
light from the other surface, what color
will you see?
– The complementary color, yellow!
Iridescence from Thin Films
Same principles as Single-Color
Interference
The shapes of the fringes for
both are made by the
differences in thickness of the
materials.
Except we are using light of mixed
frequencies and our fringes are made
of different colors.
Incoherent Light
• Light emitted by a
common lamp is
incoherent. It has
many phases of
vibration as well as
many frequencies.
• Incoherent light
spreads out after a
short distance and
loses intensity.
Coherent Light
• A beam of light that has the same
frequency, wavelength, phase, and
direction is called coherent.
• There is no interference of waves within
the beam and the beam will not spread
out and diffuse.
Laser Light
• Laser light is coherent.
• “LASER” = Light Amplification by Stimulated
Emission of Radiation
The Laser
In a laser, a light
wave emitted from
one atom stimulates
the emission of light
from a neighboring
atom so that the
crests of each wave
coincide. Thus a
coherent beam.
The Hologram
• The three-dimensional version of a
photograph produced by
interference patterns of laser
beams.
The Hologram
• The interference of the laser beams
produces fringe patterns on the
photographic plate that record the depth of
the surface of an object.
The Hologram
• The fringe pattern of a hologram
diffracts light to produce wave fronts
identical to the wave fronts given by
the object.
The Hologram
So you see the 3-D image
due to the way the
hologram diffracts light
and the way this
diffracted light
constructively and
destructively interfere. In
this way, holograms are
like diffraction gratings.
The Hologram
• Every part of the
hologram receives
and records light
from the entire
object, so you can
cut a hologram in
half and still be able
to view the whole
image.
The Hologram
• You can magnify the
image of a hologram
by looking at it with
light that has a longer
wavelength than
which it was made.