Download Chapter 24 - apphysicswarren

Lecture Outline
Chapter 24
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Chapter 24
Physical Optics: The Wave
Nature of Light
© 2010 Pearson Education, Inc.
24.1 Young’s Double-Slit Experiment
• Light can behave as
particles or as
waves…we call this….
• In 1801, Thomas Young
used interference to
demonstrate the wave
nature of light.
• Called the Young’s
Double Slit Experiment
• 2 types of
interference…they
are...they do…
• Coherent vs. incoherent
sources
24.1 Young’s Double-Slit Experiment
Passing a beam of
light through a pair
of slits produces
two beams that are
in phase, and can
interfere with each
other.
The diagram to the
left represents
Young’s double slit
experiment.
© 2010 Pearson Education, Inc.
24.1 Young’s Double-Slit Experiment
Whether the interference is constructive or
destructive depends on the path length
difference from the slits to the screen.
© 2010 Pearson Education, Inc.
24.1 Young’s Double-Slit Experiment
To calculate the path length the waves have
traveled:
© 2010 Pearson Education, Inc.
24.1 Young’s Double-Slit Experiment
The condition for the location of an interference
maximum is then (for constructive interference)
Here, n is called the order of the maximum.
The lateral distance between maxima and
central maximum for small angles is,
© 2010 Pearson Education, Inc.
24.1 Young’s Double-Slit Experiment
• In a lab experiment, monochromatic light passes through
2 narrow slits that are 0.05 mm apart. The interference
pattern is observed on a white wall 1.0 m from the slits,
and the second order maximum is at an angle of 1.5
degrees.
– a) If the slit separation decreases, the second order maximum
will be seen at an angle of greater than 1.5, at 1.5, or less than
1.5
– b) What is the wavelength of light and what is the distance
between second order and third order maxima?
– c) If d = 0.040 mm, what is the angle for the second order?
24.2 Thin-Film Interference
If light reflects from a medium of higher index of
refraction, there is a 180° phase shift. If it
reflects from a medium of lower index of
refraction, there is no phase shift.
Look at the pictures….you’ve seen this before!
© 2010 Pearson Education, Inc.
24.2 Thin-Film Interference
In determining whether the interference will be
constructive or destructive, we must look at the
path length and the angle of incidence.
If the waves are out of phase - destructive
interference
If the waves are in phase – constructive
interference
Indices of Refraction determine in/out of phase
© 2010 Pearson Education, Inc.
24.2 Thin-Film Interference
The colors of an oil spill come from
interference of waves reflecting from the
surface and from the oil–water interface.
© 2010 Pearson Education, Inc.
24.2 Thin-Film Interference
Thin-film interference is also
useful for determining the
flatness of a reflecting
surface.
If the surface is not flat, the
bands will not be straight
and even.
These are called Optical
Flats.
© 2010 Pearson Education, Inc.
24.2 Thin-Film Interference
A similar technique
can be used to
check lenses. If the
lens is not shaped
properly, the bands
will be distorted.
Newton’s Rings
© 2010 Pearson Education, Inc.
24.3 Diffraction
• We like to think that light moves in straight
line paths (rays). If this were true, we
wouldn’t be able to see interference.
• But WE DO!!! So…this means light must
deviate from straight line paths!
• As waves pass through slits, they spread
out. This spreading of light is called
diffraction.
24.3 Diffraction
• Figure 24.12 in Book
– When the width of the opening is much larger
than the wavelength of the wave there will be
little diffraction.
– When the wavelength and width are around
the same size (as the width of the slit
becomes smaller), there will be noticeable
diffraction.
24.3 Diffraction
Diffraction patterns are created when waves
encounter obstacles or openings, or are very
close to the edges of objects.
© 2010 Pearson Education, Inc.
24.3 Diffraction
Again, path length differences from different
parts of the object or opening lead to
interference.
The condition for diffraction minima is:
The diffraction pattern will be wider for
longer wavelengths and narrower slits.
The central maximum is twice as wide as
the side maxima; its width is:
© 2010 Pearson Education, Inc.
24.3 Diffraction
• When you drive through a city or
mountainous areas, the quality of your radio
reception varies sharply from place to place,
with stations seeming to fade out and
reappear. Could diffraction be a cause of this
phenomenon? Which of the following
frequency bands would you expect to be
least affected:
– Weather (162 MHz)
– FM (88 – 108 MHz)
– AM (525 – 1610 kHz)
24.3 Diffraction
• Monochromatic light passes through a slit
whose width is 0.05 mm.
• a) The resulting diffraction pattern is
generally wider for longer wavelengths,
wider for shorter wavelengths, or the same
width for all wavelengths?
• b) At what angle will the third order minimum
be seen and what is the width of the central
maximum on a screen located 1.0 m from the
slit, for a wavelength of 400 nm and 550 nm.
24.3 Diffraction
A diffraction
grating is a series
of small slits with
uniform spacing.
The maxima
become quite
narrow as the
number of slits
increase. The
minima become
wider.
© 2010 Pearson Education, Inc.
24.3 Diffraction
Diffraction gratings combine multiple-slit
interference with single-slit diffraction,
yielding the observed pattern.
© 2010 Pearson Education, Inc.
24.3 Diffraction
• Two parameters define a diffraction grating:
– The slit separation (d) [This is also called the
grating constant]
– The slit width
• Let’s look back at slide #20
24.3 Diffraction
The maxima of a diffraction grating are the
same as for a double slit:
If the light falling on a diffraction grating is
not monochromatic, a spectrum will be
seen, as different wavelengths will have
peaks at different places. A familiar example
of a (reflection) diffraction grating is a CD.
The grating constant be found with…
© 2010 Pearson Education, Inc.
24.3 Diffraction
• A particular diffraction grating produces an
n = 2 spectral order at an angle of 32 degrees
for light with a wavelength of 500 nm.
– a) How many lines per centimeter does the grating
have?
– b) At what angle can the n = 3 spectral order be
seen?
– c) What is the highest order maximum that can be
observed?
24.4 Polarization
Polarization refers to
the orientation of the
electric and magnetic
fields in an
electromagnetic wave.
This only happens in
transverse waves.
© 2010 Pearson Education, Inc.
24.4 Polarization
In unpolarized light, the electric fields have
random directions.
In partially polarized light, the electric fields are
clustered around a preferred orientation.
In fully polarized light, the electric fields are all
in the same direction.
© 2010 Pearson Education, Inc.