Light and Reflection
Overview
14-1 Characteristics of Light – identifies the components of the
electromagnetic spectrum, relates their frequency and wavelength
to the speed of light, and introduces the relationship between
brightness and distance for a light source
14-2 Flat Mirrors – applies the laws of reflection to plane mirrors
and uses ray diagrams to determine image location
14-3 Curved Mirrors – shows how image location and
magnification are calculated for concave and convex mirrors, uses
ray diagrams to confirm calculated results, and explains spherical
aberration
14-4 Color and Polarization – investigates additive and subtractive
colors and explores the phenomenon of polarization
14-1 Characteristics of Light
Electromagnetic Waves
The spectrum includes more than visible light
Objectives
“White” light can be separated into six elementary colors of the
visible spectrum: red, orange, yellow, green, blue, and violet.
White light contains a much wider spectrum than the one we can see.
Other types of radiation—inculding X rays, microwaves, and radio
waves—have many of the same properties as visible light.
Identify the components of the electromagnetic
spectrum.
Calculate the frequency or wavelength of
electromagnetic radiation.
These are all examples of electromagnetic waves.
We will use the wave model of light in our discussion.
Recognize that light has a finite speed.
Describe how the brightness of a light source is affected
by distance.
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D1
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Electromagnetic Waves
Electromagnetic waves vary depending on
frequency and wavelength
electromagnetic wave – a transverse wave consisting of oscillating
electric and magnetic fields at right angles to each other
Electromagnetic waves are distinguished by their different
frequencies and wavelengths.
In visible light, different colors have
different frequencies and wavelengths.
Visible light and invisible electromagnetic radiation have different
frequencies and wavelengths.
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Electromagnetic Waves
All electromagnetic waves move at the speed of light
All forms of electromagnetic radiation travel at a single high speed in a
vacuum.
Speed of light in a vacuum:
2.997 924 58 × 108 m/s
Speed of light in air:
2.997 09 × 108 m/s
Speed of light in physics book: 3.00 × 108 m/s
Electromagnetic Waves
Sample Problem 14A – Electromagnetic waves
The AM radio band extends from 5.4 × 105 Hz to 1.7 × 106 Hz. What are
the longest and shortest wavelengths in this frequency range?
Solution
Given:
f1 = 5.4 × 105 Hz
c = 3.00 × 108 m/s
Unknown:
Wave Speed Equation
c=f
speed of light = frequency × wavelength
f2 = 1.7 × 106 Hz
c=f
1
=
c
f
=?
2
1
2
=?
8
c
3.00×10 m/s
2
=
=
= 5.6×10 m
5
f1
5.4×10 Hz
=
8
c
3.00×10 m/s
= 1.8×10 2 m
=
6
f2
1.7×10 Hz
D3
Electromagnetic Waves
Waves can be approximated as rays
A wave can be represented by a straight line perpendicular to the wave
front.
This line is called a ray.
Brightness decreases by the square of the
distance from the source
14-2 Flat Mirrors
Objectives
Distinguish between specular and diffuse reflection of light
Apply the law of reflection for flat mirrors
Describe the nature of images formed by flat mirrors
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Reflection of Light
Reflection of Light
D1
reflection – the turning back of an electromagnetic wave at the
surface of a substance
The texture of a surface affects how it reflects light
The smoothness of a surface affects how light is reflected from it.
The reflection of light in many different directions from rough,
textured surfaces is called diffuse reflection.
reflection.
The reflection of light in only one direction from smooth, shiny
surfaces is called specular reflection.
reflection.
Smooth surface – one whose surface variations are small compared
with the wavelength of the incoming light.
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Diffuse reflection
rough, textured surface
variations large compared to
Specular reflection
smooth, shiny surface
variations small compared to
Reflection of Light
Reflection of Light
The object is being illuminated by light in
the room; a countless number of rays of
light are reflecting off the object in a
variety of directions. When viewing the
image of the object in a plane mirror, one
of these rays of light originates at the
object location and first moves along a
line towards the mirror (as represented by
the blue ray in the diagram). This ray of light is known as the
incident ray - the light ray approaching the mirror. The incident ray
intersects the mirror at the same location where your line of sight
intersects the mirror. The light ray then reflects off the mirror and
travels to your eye (as represented by the red ray in the diagram
below); this ray of light is known as the reflected ray.
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Reflection of Light
What do you think? Comment on the incorrectness of the
following diagrams. Discuss what makes them incorrect.
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To see the image of an object in a mirror, you must sight at the image.
Light will come to your eye along that line of sight.
The image location is thus located at that position where observers are
sighting when viewing the image of an object.
It is the location behind the mirror from which all the light appears to
diverge.
When each line of sight is extended backwards, each line will intersect
at the same point.
This point is the image point of the object.
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Reflection of Light
Incoming and reflected angles are equal
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In the diagram, the ray of light approaching the
mirror is known as the incident ray (Incoming
light). The ray of light which leaves the mirror is
known as the reflected ray (Reflected light). At
the point of incidence where the ray strikes the
mirror, a line can be drawn perpendicular to the
surface of the mirror; this line is known as a
normal line. The normal line divides the angle between the incident ray and the
reflected ray into two equal angles. The angle between the incident ray and the
normal is known as the angle of incidence. The angle between the reflected ray
and the normal is known as the angle of reflection. (These two angles are labeled
with the Greek letter "theta." They are read as "theta" for angle of incidence and
"theta prime" for angle of reflection.) The law of reflection states that when a ray
of light reflects off a surface, the angle of incidence is equal to the angle of
reflection. (Note that the respective angles between the incident and reflected
light rays and the mirror's surface are 90˚ - and 90˚ - '.)
Reflection of Light
Flat Mirrors
What do you think?
Image location can be predicted with ray diagrams
Consider the diagram at the right. Which
one of the angles (A, B, C, or D) is the
angle of incidence? Which one of the angles
is the angle of reflection?
A ray of light is incident towards a plane
mirror at an angle of 30-degrees with the
mirror surface. What will be the angle of
reflection?
Object distance p and image distance q are equal.
virtual image - the image that appears to be behind the mirror
Ray diagrams can be constructed using simple geometry.
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Flat Mirrors
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1. Draw diagram using proper proportions.
2. Choose a point on the object.
3. Draw a ray from the point to the mirror,
and draw its reflection. Label it as 1.
4. Draw another ray from the same point
on the object to the mirror at a different
angle than the first ray, and draw its
reflection. Label it as 2.
5. Trace both reflected rays back behind 5.the mirror until they intersect,
using dotted lines.
Flat Mirrors
Image location can be predicted with ray diagrams (cont.)
The point at which these lines intersect is the image point.
Do this for all other points on the object to locate the complete virtual
image of the object.
Note that the object's image appears as far behind the mirror as the
object is in front of the mirror (p = q), and the object height h equals
the image height h'.
Try this! Position one golf tee in front of a flat mirror. View the reflection of
the tee from three different angles. From each viewing position, draw a
line from your viewing position (where your eye is) to the point on the
mirror where you see the reflection (incidence point). Extend each line
behind the mirror (use dotted lines). Place a second tee at the point where
they intersect. Verify that this is, indeed, the image location. Measure the
angle of incidence and of reflection for each
viewing position. How does each pair of angles
compare? Measure the distance from the mirror's
surface to the tee and the image location. How do
they compare?
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14-3 Curved Mirrors
Objectives
Concave Spherical Mirrors
The Anatomy of a Curved Mirror
Calculate distances and focal lengths using the mirror equation
for concave and convex spherical mirrors
Draw ray diagrams to find the image distance and magnification
for concave and convex spherical mirrors
Distinguish between real and virtual images
Describe how parabolic mirrors differ from spherical mirrors
spherical mirrors - mirrors having the
shape of part of a sphere's surface
Spherical mirrors can be thought of as a
portion of a sphere which was sliced
away and then silvered on one of the
sides to form a reflecting surface.
Concave mirrors were silvered on the
inside of the sphere and convex
mirrors were silvered on the outside of
the sphere.
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Incoming light rays are converged by a
concave mirror and diverged by a
convex mirror.
Concave Spherical Mirrors
The Anatomy of a Curved Mirror
The Anatomy of a Curved Mirror
Principal axis – a line from the center of
the mirror, that passes through the center
of the sphere of which the mirror would
have been sliced
Center of curvature – the point in the
center of the sphere from which the
mirror was sliced; denoted by the letter
C
Vertex – the point on the mirror's
surface where the principal axis meets
the mirror; denoted by the letter A; the
geometric center of the mirror
Concave Spherical Mirrors
Focal point – a point midway from the
vertex and the center of curvature;
denoted by the letter F
Radius of curvature – the distance from
the vertex to the center of curvature;
denoted by the letter R
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Focal length – the distance from the
vertex to the focal point; denoted by the
letter f; one-half the radius of curvature;
the point in space at which light incident
towards the mirror and traveling parallel
to the principal axis will meet after
reflection
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Concave Spherical Mirrors
Concave mirrors focus light to form real images
For plane mirrors, virtual images are formed. Light does not actually
pass through the virtual image location; it only appears to an observer
as though the light was emanating from the virtual image location.
Concave mirrors are capable of producing real images (as well as
virtual images). When a real image is formed, it still appears to an
observer as though light is diverging from the real image location.
In the case of a real image, light is actually passing through the image
location.
Real image – an image formed when rays of light actually pass
through the image location (where they intersect)
Concave Spherical Mirrors
Concave mirrors focus light to form real images (cont.)
Light rays from an object (such as a light bulb) reflect off a concave
mirror, according to the law of reflection, and converge at a point,
where a replica of the object is formed (the image).
Once the light rays reach the image location, they begin to diverge.
The point at which all the reflected light rays converge is known as the
image point.
It is also the point from which
reflected light rays appear to an
observer to be diverging.
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Concave Spherical Mirrors
Concave mirrors focus light to form real images (cont.)
If the light bulb is located at a
different location, the same
principles apply.
The image location is the
location from which reflected
light appears to diverge.
By determining the path which
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light from the bulb takes after
reflecting from the mirror, the
image location can be identified.
Although the same principal applies, the image location is different,
depending upon where the object is located: if beyond C, the image is
between C and F; if between C and F, the image is beyond C.
Concave Spherical Mirrors
Two Rules of Reflection for Concave Mirrors
Light always reflects according to the law of reflection, regardless of
whether the reflection occurs off a flat surface or a curved surface.
However, it is difficult
to determine the angle
of reflection off a
curved surface.
There are two simple
rules of reflection for
concave mirrors:
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Any incident ray traveling parallel to the principal axis on the way to the
mirror will pass through the focal point upon reflection.
Any incident ray passing through the focal point on the way to the mirror
will travel parallel to the principal axis upon reflection.
Concave Spherical Mirrors
Ray Diagrams - Concave Mirrors
Steps to draw ray diagrams for concave mirrors:
Concave Spherical Mirrors
Ray Diagrams - Concave Mirrors (cont.)
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1. Pick a point on the top of the object and
draw two incident rays traveling towards the
mirror.
Using a straight edge, accurately draw one
ray so that it passes exactly through the
focal point on the way to the mirror. Draw
the second ray such that it travels exactly
parallel to the principal axis. Place
arrowheads upon the rays to indicate their
direction of travel.
2. Once these incident rays strike the mirror,
reflect them according to the two rules of
reflection for concave mirrors.
Steps to draw ray diagrams for concave mirrors:
3. Mark the image of the top of the object.
The image point of the top of the object is
the point where the two reflected rays
intersect. If you were to draw a third pair of
incident and reflected rays, then the third
reflected ray would also pass through this
point. This is merely the point where all
light from the top of the object would
intersect upon reflecting off the mirror. Of
course, the rest of the object has an image as
well and it can be found by applying the
same three steps to another chosen point.
4. Repeat the process for the bottom of the object.
Concave Spherical Mirrors
Ray Diagrams - Concave Mirrors (cont.)
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Concave Spherical Mirrors
Ray Diagrams - Concave Mirrors (cont.)
Ray diagram for object less than one focal length from mirror:
Follow same steps as before.
Light rays diverge in front of the mirror, so a virtual image is formed
behind the mirror.
Trace reflections to the point at which they converge, behind the mirror.
Image is virtual, upright, and enlarged.
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Ray diagram for object at focal point:
When an object is at the focal point of a concave mirror, the reflected
light rays are parallel to each other.
They neither converge nor diverge in front or behind the mirror.
No image is formed.
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Concave Spherical Mirrors
Image Characteristics for Concave Mirrors, Summarized
Five cases:
Concave Spherical Mirrors
Image Characteristics for Concave Mirrors, Summarized (cont.)
Case 1: The object is located beyond C
Case 1: the object is located beyond the center of curvature (C)
Case 2: the object is located at the center of curvature (C)
Case 3: the object is located between the center of curvature (C) and the
focal point (F)
Case 4: the object is located at the focal point (F)
Case 5: the object is located in front of the focal point (F)
Image location is between C and F
Image is real, inverted, reduced in size
Case 2: The object is located at C
Image location is at C
Image is real, inverted, same size as object
Case 3: The object is located between C and F
Image location is beyond C
Image is real, inverted, larger than object
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Concave Spherical Mirrors
Image Characteristics for Concave Mirrors, Summarized (cont.)
Case 4: The object is located at F
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Concave Spherical Mirrors
Image location can be predicted with the mirror equation
The mirror equation relates the object distance, image distance, and
focal length of a spherical mirror.
No image is formed
Mirror Equation
1
1
1
+
=
p
q
f
Case 5: The object is located in front of F
Image location is behind mirror
Image is virtual, upright, larger than object
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Image locations for various object
locations relative to C and F.
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1
1
1
+
=
object distance
image distance
focal length
Distances are positive in front of and negative behind the mirror.
Object and image heights are positive above and negative below the
principal axis.
Concave Spherical Mirrors
Image location can be predicted with the mirror equation (cont.)
Concave Spherical Mirrors
Magnification relates image and object sizes
Magnification, M, is defined as the ratio of the height of the image to
the height of the object.
M is also the negative of the ratio of the image distance to the object
distance.
Equation for Magnification
M =
Holt Physics
magnification =
h'
=
h
q
p
image height
=
object height
image distance
object distance
Holt Physics
Concave Spherical Mirrors
Magnification relates image and object sizes (cont.)
Image in front of mirror: M is negative; image is inverted.
Image behind mirror: M is positive; image is upright.
Concave Spherical Mirrors
Sample Problem 14B – Concave mirrors
A concave spherical mirror has a focal length of 10.0 cm. Locate the
image of a pencil that is placed upright 30.0 cm from the mirror. Find
the magnification of the image. Draw a ray diagram to confirm your
answer.
Solution
Given:
f = +10.0 cm
p = +30.0 cm
Unknown: q = ?
M=?
Diagram: (on board)
1
1
1
+
=
Choose an expression containing the unknown q:
p
q
f
Concave Spherical Mirrors
Sample Problem 14B – Concave mirrors (cont.)
Isolate the unknown so that you can solve for it:
1 1
1
1
0.100 0.033
0.067
1
=
=
=
=
q
f
p
10.0 cm 30.0 cm
1 cm
1 cm
1 cm
q = 15 cm
Choose an expression containing the unknown M:
M = -
q
p
q
15 cm
= = - 0.50
p
30.0 cm
Convex Spherical Mirrors
The Anatomy of a Curved Mirror
The focal point, center of curvature, and
principal axis are all located behind a
convex mirror.
A convex mirror is said to have a
negative focal length.
Convex mirrors diverge incoming light
rays.
Reflected light rays will never intersect
on the object side of the mirror.
Convex mirrors produce virtual images
that are located somewhere behind the
mirror.
Classroom Practice – Concave mirrors
When an object is placed 30.0 cm in front of a concave mirror, a real
image is formed 60.0 cm from the mirror's surface. Find the focal
length.
A square object is placed 15 cm in front of a concave mirror with a focal
length of 25 cm. A round object is placed 45 cm in front of the same
mirror. Find the image distance, magnification, and type of image
formed for each object. Draw ray diagrams for each object to confirm
your answers.
And solve:
M = -
Concave Spherical Mirrors
Convex Spherical Mirrors
Regardless of the viewing
angle, the reflected light rays
seem to be diverging from a
point behind the mirror, the
virtual image location.
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Recall that an image is the
location in space from which it
appears that light diverges.
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Convex Spherical Mirrors
Two Rules of Reflection for Convex Mirrors
Ray Diagrams - Convex Mirrors
There are two simple rules of reflection for convex mirrors:
Any incident ray traveling parallel to the principal axis on the way to a
convex mirror will reflect in a manner that its extension will pass
through the focal point.
Any incident ray traveling towards a convex mirror such that its
extension passes through the focal point will reflect and travel parallel
to the principal axis.
Convex Spherical Mirrors
Ray Diagrams - Convex Mirrors (cont.)
Steps to draw ray diagrams for convex mirrors:
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1. Pick a point on the top of the object and
draw two incident rays traveling towards the
mirror.
Using a straight edge, accurately draw one
ray so that it travels towards the focal point
on the opposite side of the mirror; this ray
will strike the mirror before reaching the
focal point; stop the ray at the point of
incidence with the mirror. Draw the second
ray such that it travels exactly parallel to the
principal axis. Place arrowheads upon the
rays to indicate their direction of travel.
Convex Spherical Mirrors
Ray Diagrams - Convex Mirrors (cont.)
Steps to draw ray diagrams for convex mirrors:
Steps to draw ray diagrams for convex mirrors:
2. Once these incident rays strike the mirror,
reflect them according to the two rules of
reflection for convex mirrors.
The ray that travels towards the focal point will
reflect and travel parallel to the principal axis.
Use a straight edge to accurately draw its path.
The ray which travels parallel to the principal
axis on the way to the mirror will reflect and
travel in a direction such that its extension
passes through the focal point. Align a straight
edge with the point of incidence and the focal
point, and draw the second reflected ray. Place
arrowheads upon the rays to indicate their
direction of travel. The two rays should be
diverging upon reflection.
Convex Spherical Mirrors
3. Locate and mark the image of the top of the
object.
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The image point of the top of the object is the point
where the two reflected rays intersect. Since the two
reflected rays are diverging, they must be extended
behind the mirror in order to intersect. Using a
straight edge, extend each of the rays using dashed
lines. Draw the extensions until they intersect. The
point of intersection is the image point of the top of
the object. Both reflected rays would appear to
diverge from this point. This is merely the point from
which all light from the top of the object would
appear to diverge upon reflecting off the mirror. Of
course, the rest of the object has an image as well and
it can be found by applying the same three steps to
another chosen point.
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Convex Spherical Mirrors
Ray Diagrams - Convex Mirrors (cont.)
Image Characteristics for Convex Mirrors
Steps to draw ray diagrams for convex mirrors:
For convex mirrors, the image is always
located behind the convex mirror
a virtual image
an upright image
reduced in size (i.e., smaller than the object)
4. Repeat process for bottom of object.
The goal of a ray diagram is to determine the
location, size, orientation, and type of image which is
formed by the convex mirror. Typically, this requires
determining where the image of the upper and lower
extreme of the object is located and then tracing the
entire image. After completing the first three steps,
only the image location of the top extreme of the
object has been found. Thus, the process must be
repeated for the point on the bottom of the object. If
the bottom of the object lies upon the principal axis
(as it does in this example), then the image of this
point will also lie upon the principal axis and be the
same distance from the mirror as the image of the top
of the object. At this point the complete image can be
filled in.
Convex Spherical Mirrors
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Convex Spherical Mirrors
Image Characteristics for Convex Mirrors (cont.)
As the object distance is decreased, the image distance is decreased
and the image size is increased.
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Spherical Mirrors
What do you think?
The diagram shows a spherical surface which is silvered on both sides.
Thus, the surface serves as double-sided mirror, with one of the sides
being the concave and one being the convex side. The principal axis,
focal point, and center of curvature are shown. The region on both
sides of the mirror is divided into eight sections (labeled M, N, P, Q, R,
S, T, and W). Five objects (labeled 1, 2, 3, 4, and 5) are shown at
various locations about the double-sided mirror. Use the diagram to
answer the questions #1-6.
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Spherical Mirrors
Convex Spherical Mirrors
Sample Problem 14C – Convex Mirrors
An upright pencil is placed in front of a convex spherical mirror with a
focal length of 8.00 cm. An erect image 2.50 cm tall is formed 4.44 cm
behind the mirror. Find the position of the object, the magnification of
the image, and the height of the pencil.
Solution
Given:
f = – 8.00 cm
q = – 4.44 cm
h' = 2.50 cm
Unknown: p = ?
M=?
h=?
Diagram: (on board)
1
1
1
+
=
Choose an expression containing the unknown p:
p
q
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Convex Spherical Mirrors
Sample Problem 14C – Convex Mirrors (cont.)
Isolate the unknown so that you can solve for it:
1 1
1
1
1
- 0.125 - 0.225 0.100
= - =
=
=
p f q - 8.00cm - 4.44 cm
1 cm
1cm
1 cm
p = 10.0 cm
Choose an expression containing the unknown M:
q
M = p
And solve:
M = -
- 4.44 cm
q
== 0.444
p
10.0 cm
Convex Spherical Mirrors
Sample Problem 14C – Convex Mirrors (cont.)
Choose an expression containing the unknown h':
M =
h'
=
h
q
p
Rearrange and solve:
h=-
10.0 cm
p
2.50 cm = 5.63 cm
h' = q
- 4.44 cm
f
Convex Spherical Mirrors
Classroom Practice – Convex Mirrors
The radius of curvature of a convex mirror is 12.0 cm. Where is the
focal point located?
Find the position of the image for an object placed at the following
distances from the mirror in the previous question: p = 1.00 cm, 2.00
cm, 3.00 cm, 6.00 cm, 12.0 cm, 30.0 cm, 50.0 cm
Convex Spherical Mirrors
Classroom Practice – Convex Mirrors (SOLUTION)
The radius of curvature of a convex mirror is 12.0 cm. Where is the
focal point located?
The focal point is located halfway between the vertex and center of
curvature of the mirror. That is, 6.00 cm behind the mirror (f = – 6.00
cm).
Find the position of the image for an object placed at the following
distances from the mirror in the previous question: p = 1.00 cm, 2.00
cm, 3.00 cm, 6.00 cm, 12.0 cm, 30.0 cm, 50.0 cm
Use the mirror equation:
How does the position of the image vary as the object in the previous
question moves farther away from the mirror?
Convex Spherical Mirrors
Classroom Practice – Convex Mirrors (SOLUTION)
How does the position of the image vary as the object in the previous
question moves farther away from the mirror?
The image is always behind the mirror and between the mirror and the
focal point. It moves from q = 0 to q = f as the object moves away from
the mirror (from p = 0 to infinity).
1
1
1
+
=
p
q
f
q = –0.855 cm, –1.50 cm, –2.00 cm, –2.99 cm, –4.00 cm, –5.00 cm, –5.35
cm
Parabolic Mirrors
Parabolic mirrors eliminate spherical aberration
spherical aberration – the effect, present in spherical mirrors, that
occurs when parallel light rays far from the principal axis converge
away from the mirror's focal point, causing a blurred image
Ways to reduce the effect of spherical aberration are to use a small
spherical mirror, limit the portion of a larger mirror that is used, or use
a parabolic mirror.
All rays parallel to
the principal axis
converge at a
parabolic mirror's
focal point.
Holt Physics
Parabolic Mirrors
14-4 Color and Polarization
Reflecting telescopes use parabolic
mirrors
Objectives
Recognize how additive colors affect the color of light
Reflecting telescopes employ a
parabolic mirror (called an objective
mirror) to focus light.
One type of reflecting telescope, called
a Cassegrain reflector, is shown.
Recognize how pigments affect the color of reflected light
Explain how linearly polarized light is formed and detected
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Color
Objects absorb certain wavelengths of light and reflect the rest.
The color of an object depends on which wavelengths shine on the
object and which wavelengths are reflected.
Color
The color of an object is not actually within the object itself.
The color is in the light which shines upon it that ultimately becomes
reflected to our eyes.
So if an object absorbs all of the frequencies of visible light except for
the frequency associated with green light, then the object will appear
green in the presence of ROYGBIV.
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Holt Physics
Color
Color
Additive primary colors produce white light when combined
Additive primary colors produce white light when combined
White light can be split into its elementary colors (ROYGBIV).
Red and green make yellow.
Those elementary colors can be
recombined to form white light.
Red, green, and blue are called
the additive primary colors, and
can be combined to form white
light.
They can also be combined in
varying proportions to form all
the other colors of the spectrum.
Red and blue make magenta.
Green and blue make cyan.
Yellow and blue make white.
Yellow is the complementary
color of blue.
Two primary colors combine to
form the complement of the third
primary color.
Color
Color
Additive primary colors produce
white light when combined
Subtractive primary colors filter out all light when combined
Pigments absorb certain wavelengths of light and reflect the rest,
effectively subtracting certain
colors from the light.
When pigments are mixed, each
one subtracts certain colors from
white light, and the resulting
color depends on the frequencies
that are not absorbed.
Applications of additive primary colors
are coloring glass and producing images
on a color television.
TVs use small, colored dots of light
(primary colors) called pixels. Varying
the amount and intensity of color in each
pixel allows all colors to be displayed.
The primary pigments (or
primary subtractive colors) are
cyan, magenta, and yellow.
Humans can see color because there are
three kinds of color receptors (cone cells)
in the eye, sensitive to either red, green,
or blue light.
Holt Physics
Color
Color
Subtractive primary colors filter out all light when combined
Consider white light shining on a shirt. If white light is shining on the
shirt, then RGB is shining on it. If the shirt absorbs blue light, then
only red and green light will be reflected from the shirt. The shirt will
appear yellow. This illustrates the process of color subtraction.
The ultimate color appearance of an object is determined by beginning
with a single color or mixture of colors and identifying what color or
colors of light are subtracted from the original set.
Subtractive primary colors filter out all light when combined
Now suppose that cyan light is shining on the same shirt - a shirt made
of a material which is capable of absorbing blue light.
What appearance will such a shirt have if illuminated with cyan light
and how can we account for its appearance?
Apply the process of color subtraction.
C - B = (G + B) - B = G
The shirt will appear green.
W - B = (R + G + B) - B = R + G = Y
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Color
Subtractive primary colors filter out all light when combined
Try it!
Test your understanding of these principles of color subtraction by
determining the color appearance of the same shirt if illuminated with
other colors of light. Be sure to begin by determining the primary
color(s) of light which are incident upon the object and then
subtracting the absorbed color from the incident color(s).
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Color
Subtractive primary colors filter out all light when combined
Subtractive primary colors are complementary to the primary colors.
The color of light absorbed by a pigment is merely the complementary
color of that pigment.
Color
Subtractive primary colors filter out all light when combined
Try it!
Magenta light shines on a sheet of paper containing a yellow pigment.
Determine the appearance of the paper.
M - B = (R + B) - B = R
Yellow light shines on a sheet of paper containing a red pigment.
Determine the appearance of the paper.
Y - C = (R + G) – (B + G) = R
Yellow light shines on a sheet of paper containing a blue pigment.
Determine the appearance of the paper.
Y - Y = (R + G) - (R + G) = No reflected light = Black
Holt Physics
Polarization of Light Waves
What is polarization?
Polarization of Light Waves
What is polarization? (cont.)
The vibrations of an electromagnetic wave occur
in more than one plane of vibration.
A light wave which is vibrating in more than one
plane is referred to as unpolarized light.
Light emitted by the sun, by a lamp in the
classroom, or by a candle flame is unpolarized
light.
It is helpful to picture unpolarized light as a wave
which has an average of half its vibrations in a
horizontal plane and half of its vibrations in a
vertical plane.
It is possible to transform unpolarized light into polarized light.
Polarized light waves are light waves in which the vibrations occur in a
single plane.
The process of transforming unpolarized light into polarized light is
known as polarization.
Some of the methods of polarizing light are transmission, reflection,
and scattering.
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Polarization of Light Waves
Light can be polarized through transmission
The most common
method of polarization
involves the use of a
Polaroid filter.
Polaroid filters are made
of a special material
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which is capable of blocking
one of the two planes of vibration of an electromagnetic wave.
A Polaroid serves as a device which filters out one-half of the
vibrations upon transmission of the light through the filter.
When unpolarized light is transmitted through a Polaroid filter, it
emerges with one-half the intensity and with vibrations in a single
plane; it emerges as polarized light.
Polarization of Light Waves
Light can be polarized through transmission (cont.)
The vertical vibration is free to
pass between vertically oriented
slats in the fences.
A vertical vibration cannot pass
through when the slats are
oriented horizontally.
Polarization of Light Waves
Light can be polarized through transmission (cont.)
A polarization filter has a polarization axis (transmission axis). This
polarization axis extends across the length of the filter and only allows
vibrations of the electromagnetic wave that are parallel to the axis to
pass through. Any vibrations which are perpendicular to the
polarization axis are blocked by the filter.
Polarization of Light Waves
Light can be polarized by reflection
Unpolarized light can also
undergo polarization by reflection
off of nonmetallic surfaces.
Metallic surfaces reflect light
with a variety of vibrational
directions; such reflected light is
unpolarized.
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Nonmetallic surfaces such as
asphalt roadways, snow fields and water reflect light such that there is
a large concentration of vibrations in a plane parallel to the reflecting
surface.
In the same way, two Polaroid
filters with their polarization
axes oriented perpendicular to
each other will block all the light.
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A person viewing objects by means of light reflected off of
nonmetallic surfaces will often perceive a glare if the extent of
polarization is large.
Polarization of Light Waves
Light can be polarized by scattering
Polarization also occurs when light is scattered while traveling through
a medium.
When light strikes the atoms of a material, it will often set the
electrons of those atoms into vibration, which, in turn, produce their
own electromagnetic wave, which strikes more atoms, etc.
This absorption and reemission of light waves causes the light to be
scattered about the medium.
This scattered light is partially polarized.
Polarization by scattering is observed as light passes through our
atmosphere. The scattered light often produces a glare in the skies.