Wave Behavior
Another McGourty-Rideout
Production
The Physics of Waves
• All waves follow the laws of physics no
matter what type
• Waves can be reflected, refracted,
diffracted, absorbed, scattered and
experience interference
Reflection
• Reflection happens when a wave bounces off
an obstacle.
Specular reflection: smooth surface
– Angle of incidence = angle of reflection
Diffuse reflection: rough surface
– Reflection in all directions because angle of
incidence varies over the surface due to its
roughness
Law of Reflection
angle of incidence= angle of reflection
Reflection
• An echo example of a
reflection
• Radar uses this
principle to
determine the
size,
characteristics
of, and distance
to an object
Refraction
• Occurs as waves move from
one medium into another or
within a medium, like air, that
varies in density or
temperature
• Waves bend toward the normal
when they move from a less
optically dense medium
(faster) to a more optically
dense medium (slower)
• Waves bend away from the
normal when the opposite is
true
Snell’s Law of Refraction
n1 sin 1  n2 sin  2
• Angles are measured with respect to
the normal
Refraction
θ2
θ1
Index of Refraction
n=c/v
• nair
= 1.0003
• nwater = 1.33
• nvacuum = 1.00
• Can you explain why “n” is a naked
number? Can it ever be less than 1?
Index of Refraction Redux
n=√εμ
ε = electric permittivity
μ = magnetic permeability
These describe how the material interacts
with electric and magnetic fields
Atmospheric Refraction
• Causes gradual curve
of light from stars and
sun
• Makes sun visible 2-3
minutes before
sunrise and after
sunset
Dispersion
The index of refraction of real materials actually depends on the
frequency of the light being bent.
Dispersion is the explanation for rainbows:
Each color has its own frequency 
Each gets slowed down differently 
Each comes out at a different angle
Diffraction
• Waves that have
longer wavelengths,
or lower frequencies,
diffract better than
high frequency waves
• Diffraction patterns
are determined by
both the size of the
opening and the
wavelength
Absorption
• Absorption happens when the medium has the
ability to absorb the energy of the wave
• When the wave is absorbed, its energy is
transferred to the medium and the wave is
gone
• Gradual absorption as the wave penetrates
the medium is called “attenuation”
• Absorption of only specific frequencies will
leave “gaps” in the continuing wave spectrum
called “spectral absorption lines”
Absorption
• Absorption at the
quantum level happens
when an individual photon
has the exact energy that
corresponds to an energy
gap between two energy
states of the medium
• The type of energy gap
corresponds directly to the
frequency of the photon
Scattering
• If the photon is absorbed and then re-emitted
immediately, it is said to be scattered
• How the light is scattered is dependent on the
frequency of the light and the size of the
particle it is scattering from
• Some of the energy of the light is absorbed by
the scatterer and so the re-emitted light has a
little less energy
Scattering
• If the photon has a longer wavelength than the
size of the scattering particle, it is called
Rayleigh scattering
• In Rayleigh scattering the very long wavelength
light is hardly scattered at all but the shorter
wavelength is much more strongly scattered
• Since blue light is much ‘shorter’ than red, it gets
more scattered by the molecules in the air:
therefore the sky is blue!
Interference
• When two or more waves come together,
they “superimpose” or add together
• The total amplitude is simply the sum
(positive & negative!) of all the individual
amplitudes
• The extremes of what can happen are
called constructive interference and
destructive interference
Constructive and Destructive Algebraic
Addition
S
S
Partially Constructive
(somewhat out of phase)
Constructive
(in phase)
S
Destructive
(180° out of phase)
S
Non-coherent signals
(noise)
Interference Fringes
• Interference fringes
are a series of bright
and dark bands
• Sometimes straight,
sometimes circular,
sometimes more
complicated
Young’s Double Slit Experiment
• Light diffracting
through 2 slits
produces
fringes on a
screen
• Bright fringes
are areas of
constructive
interference
• Dark fringes are
areas of
destructive
interference