Diffraction and Interference
Physics 2212
Light
• Light has Wave properties
• Light can Diffract
• Light can Interfere
–Constructively
–Destructively
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
a) Wave fronts are made up of tinier wave fronts.
b) Every point on any wave front is a new source for a
secondary wave front.
Diffraction
Any bending of a wave around an obstacle or edges
of an opening by means other than reflection or
refraction.
This gap is less than λ
This gap is greater than λ
Cont..
Light waves bend due to the lack
of a sharp shadow on the wall
Water waves easily bend
around obstacles
Diffraction- Limits of Visibility
• Diffraction is a limiting
factor 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
• Bright fringes = constructive interference
– Waves arrive at the screen in phase
• Dark fringes = destructive interference
– Waves arrive at the screen out of phase
Diffraction Grafting
A diffraction grating
consists of thousands
of parallel slits etched
on glass so that
brighter and sharper
patterns can be
observed than with
Young’s experiment.
Equation is similar.
d sin q
d
q
d sin q = nl
n = 1, 2, 3, …
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.
Cont…
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.
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.
• 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.
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
• 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 interference of the laser beams produces
fringe patterns on the photographic plate that
record the depth of the surface of an object.
• The fringe pattern of a hologram diffracts light
to produce wave fronts identical to the wave
fronts given by the object.
Cont..