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Light and Reflection
Chapter 13
Page 444
Characteristics of Light
—  Let’s talk about the electromagnetic spectrum.
—  This includes visible light. What looks like “white”
light can be split into many different colors.
—  Light possesses characteristics of both particles
and waves. For our purposes, we will model light as
a wave.
—  Just like other waves, electromagnetic waves vary
depending on their frequency and wavelength.
Characteristics of Light
—  Differences in frequency and wavelength account
for the different colors we see, as well as whether or
not the wave is even visible.
—  Light is composed of oscillating electric and
magnetic fields, which are perpendicular to each
other, and perpendicular to the direction of the
wave’s motion.
—  There’s a table of different waves on the EM
spectrum on page 447. Wavelength, λ, is
measured in units of length, like m, mm, cm.
Frequency is measured in Hertz.
Characteristics of Light
— 
Characteristics of Light
—  ALL electromagnetic waves move at the speed of
light!
—  In a vacuum: 2.99792458 x 108 m/s.
—  In air: 2.99709 x 108 m/s.
—  There is a relationship between frequency,
wavelength, and speed that we’ve seen before as
regards waves. This applies to light waves, as well!
—  c=fλ
Sample Problem A page 448
Practice A page 449
Characteristics of Light
—  The motion of electromagnetic waves can be
approximated as rays.
—  In reality, light spreads out as it travels.
—  A laser is an example of a light source that spreads
very little; it is concentrated and focused, unlike—
for example—our fluorescent lights.
Characteristics of Light
—  Illuminance decreases as the square of the
distance from the source.
—  The rate at which light is emitted from a source is
called the luminous flux and is measured in
lumens.
—  My dad will talk for hours about lumens. He used
to work for Sylvania. He once had to travel by air
with a xenon lamp for Imax movies, but this was
before 9/11 so security just asked some questions
and didn’t hassle him about it. I’ve heard this story
fifty times. Zzzzzzz…..............
Characteristics of Light
—  The illuminance—the luminous flux divided by the
area of the surface—is measured in lm/m2 which is
called lux. It decreases as the radius squared when
you move away from a light source.
Section Review page 450
Flat Mirrors
—  Light interacts with surfaces in two ways: some of the
light may be absorbed, and some of the light may be
reflected.
—  No surface is a perfect reflector, though good mirrors
can reflect around 90% of incident light.
—  A rougher surface reflects incoming light in many
different directions. This is called diffuse reflection.
—  A smooth surface reflects incoming light in one
direction. This is called specular reflection. (The
direction depends on the direction of incoming light.)
Flat Mirrors
—  Incoming and reflected angles are equal.
—  Imagine a straight line drawn perpendicular to the
surface at the point where the incoming light is
striking it. (Another word for this kind of line is
normal.)
—  The angle of incidence is measured between the
ray of incoming light and the normal line.
—  The angle of reflection is measured between the
normal line and the ray of reflected light.
Flat Mirrors
—  The simplest mirror is a flat mirror.
—  Light bounces off objects in front of the mirror and
is reflected to an observer. To the observer, the
light originates on the other side of the mirror.
Flat Mirrors
—  An object’s image is said to be at a location behind
the mirror.
—  The image will be the same distance from the
mirror as the object. The image is also the same
size as the object.
—  This image is known as a virtual image. It can
never be displayed on a physical surface.
Flat Mirrors
—  We can predict an image location by using
geometric ray diagrams.
—  Sizes and distances will be reflected symmetrically
in a flat mirror. The object’s distance from the
mirror at any given point will be the same as the
image’s distance from the mirror. All dimensions in
the image will be the same as on the object.
Curved Mirrors
—  While flat mirrors create images with the same
dimensions as the original object, curved mirrors
create images with different dimensions compared
to the original object.
—  A basic type of curved mirror is a concave
spherical mirror—a mirror that is shaped like part
of a sphere’s surface. Concave mirrors like these
can magnify nearby objects, as needed, and are
often used when applying makeup, for example.
Curved Mirrors
—  As you get farther away from a concave spherical
mirror, the image will appear smaller and upsidedown.
—  The location of the image is determined in part by
the radius of curvature of the mirror’s surface, R.
The radius of curvature is the distance from the
mirror’s surface to the center of curvature, C.
—  A mirror’s principal axis extends from the center of
the mirror’s surface through its center of curvature.
Curved Mirrors
—  Rays that are farther away from the principal axis
don’t exactly intersect at the image point.
—  This is called spherical aberration.
—  Ray diagrams and the mirror equations are valid for
rays that are near the principal axis of the mirror.
—  These are called paraxial rays. (Para—near, axial—
axis.)
Curved Mirrors
—  The Mirror Equation: If you know the mirror radius
and the distance the object is from the mirror, you
can predict where the image will appear using the
mirror equation.
—  Object distance: p
—  Image distance: q
—  Radius of curvature: R
—  Focal length: f
Curved Mirrors
—  Consider light coming from very far away from the
mirror. From large enough distances, we say that
the light is coming “from infinity.”
—  Light coming from infinity will be directed towards
the same location, called the focal point—halfway
between the mirror and its center of curvature.
—  Real images are formed on the front side of the
mirror.
—  Virtual images are formed behind the mirror, on the
back side.
Curved Mirrors
—  The image from a curved mirror is rarely the same
size as the object. Thus, we can talk about the
magnification of an image.
—  Once you know the object’s image location, you can
determine its magnification.
—  Magnification: M
—  Object height: h
—  Image height: h’
Curved Mirrors
—  For negative values of M, the image will be upsidedown.
—  For values of M between 0 and 1, the image will be
smaller.
—  For values of M larger
than 1, the image will
be larger.
Curved Mirrors
—  Another type of curved mirror is a convex spherical
mirror.
—  These are also called diverging mirrors, as all rays
coming from infinity will bounce off at a greater
angle away from the principal axis.
—  The focal length for a convex spherical mirror is
negative; the focal point and center of curvature
are behind the mirror.
—  Think: fisheye mirrors opposite driveways; smaller
insets in rear-view mirrors.
Sample Problem C page 465
Practice C page 466
Curved Mirrors
—  In our ray diagrams for spherical mirrors, you may
have noticed that our rays rarely intersect perfectly
at the image’s location. This is a real phenomenon
called a spherical aberration, and when you see
images reflected in spherical mirrors they will
appear blurry.
—  A mirror whose surface cross-section is parabolic
eliminates this problem completely!
Curved Mirrors
—  Parabolic mirrors are used in some telescopes!
Color and Polarization
—  Objects can absorb certain wavelengths of light
while reflecting others.
—  We perceive the reflected wavelengths of light. For
example, a fresh, green leaf reflects green light.
Color and Polarization
—  Colors of light can be additive. Additive primary
colors of light—red, green, and blue—will combine
to make white light.
—  Additive properties of colors are applied in
television screens and projections.
—  Colors can be subtractive. Subtractive primary
colors of light—cyan, magenta, and yellow—will
combine to filter out all light.
—  Subtractive properties of colors can be applied in
painting, coloring, etc.
Color and Polarization
—  Light from a normal source will include various
waves that have electric fields which oscillate in
many directions.
—  This light is unpolarized.
—  Polarization is when light passes through a filter
which only allows waves with a certain orientation
to pass through. These waves have linear
polarization.
Color and Polarization
—  Light is polarized along the transmission axis of
the substance it passes through.
—  If light is shone at two substances with
perpendicular transmission axes, light will not get
through.
Color and Polarization
—  Polarization can also occur by reflection and by
scattering.
—  Light reflected off a surface will be polarized
parallel to the surface.
—  Light can be scattered off molecules of
atmospheric gas.