Transcript Unit 6, Chapter 17

CPO Science
Foundations of Physics
Unit 6, Chapter 17
Unit 6: Light and Optics
Chapter 17 Light and Color
 17.1 Reflection and Refraction
 17.2 Mirrors, Lenses, and Images
 17.3 Optical Systems
Chapter 17 Objectives
1. Describe the functions of convex and concave
lenses, a prism, and a flat mirror.
2. Describe how light rays form an image.
3. Calculate the angles of reflection and refraction for a
single light ray.
4. Draw the ray diagram for a lens and a mirror showing
the object and image.
5. Explain how a fiber-optic circuit acts like a pipe for
light.
6. Describe the difference between a real image and a
virtual image and give an example of each.
Chapter 17 Vocabulary Terms
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lens
mirror
prism
optics
geometric optics
specular
reflection
diffuse
converging
diverging
law of reflection
normal line
ray diagram
magnification
object
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index of refraction
focal point
focal length
optical axis
light ray
magnification
critical angle
Snell’s law
real image
virtual image
chromatic
aberration
 refraction
 fiber optics
 dispersion
 magnifying glass
 spherical
aberration
 reflection
 diffraction
 telescope
 focus
 total internal
reflection
 resolution
 pixel image
 focal plane
 thin lens formula
17.1 Reflection and Refraction
 Key Question:
How do we describe the
reflection and refraction
of light?
*Students read Section 17.1
AFTER Investigation 17.1
17.1 Reflection and Refraction
 The overall study of how light behaves is called optics.
 The branch of optics that focuses on the creation of
images is called geometric optics, because it is based
on relationships between angles and lines that
describe light rays.
17.1 Reflection and Refraction
 A lens is an optical device
that is used to bend light
in a specific way.
 A converging lens bends
light so that the light rays
come together to a point.
 A diverging lens bends
light so it spreads light
apart instead of coming
together.
17.1 Reflection and Refraction
 Mirrors reflect light and allow us to see ourselves.
 A prism is another optical device that can cause light to
change directions.
 A prism is a solid piece of glass with flat polished
surfaces.
17.1 Reflection
 Images appear in mirrors
because of how light is
reflected by mirrors.
 The incident ray follows the
light falling onto the mirror.
 The reflected ray follows the
light bouncing off the mirror.
17.1 Reflection
 In specular reflection each incident ray bounces off in
a single direction.
 A surface that is not shiny creates diffuse reflection.
 In diffuse reflection, a single ray of light scatters into
many directions.
Law of Reflection
The incident ray
strikes the mirror.
The reflected ray
bounces off.
The angle of
incidence equals
the angle of
reflection.
17.1 Law of reflection
30o
30o
 A light ray is incident on a plane mirror with a 30
degree angle of incidence.
 Sketch the incident and reflected rays and determine
the angle of reflection.
17.1 Refraction
 Light rays may bend as
they cross a boundary
from one material to
another, like from air to
water.
 This bending of light rays
is known as refraction.
 The light rays from the
straw are refracted (or
bent) when they cross from
water back into air before
reaching your eyes.
17.1 Refraction
When a ray of light crosses from one material to another,
the amount it bends depends on the difference in index
of refraction between the two materials.
17.1 Index of refraction
The ability of a material to bend rays of light is
described by the index of refraction (n).
17.1 Snell's law of refraction
 Snell’s law is the relationship between the angles of
incidence and refraction and the index of refraction
of both materials.
Angle of incidence
(degrees)
Angle of refraction
(degrees)
ni sin Qi = nr sin Qr
Index of
refraction of
incident
material
Index of
refraction of
refractive
material
17.1 Calculate the angle of refraction
 A ray of light traveling
through air is incident on
a smooth surface of water
at an angle of 30° to the
normal.
 Calculate the angle of
refraction for the ray as it
enters the water.
17.1 Dispersion and prisms
 When white light passes through a glass prism,
blue is bent more than red.
 Colors between blue and red are bent proportional
to their position in the spectrum.
17.1 Dispersion and prisms
 The variation in refractive
index with color is called
dispersion.
 A rainbow is an example
of dispersion in nature.
 Tiny rain droplets act as
prisms separating the
colors in the white light
rays from the sun.
17.2 Mirrors, Lenses, and Images
Key Question:
How does a lens or
mirror form an image?
*Students read Section 17.2
AFTER Investigation 17.2
17.2 Mirrors, Lenses, and Images
We see a world of images created on the retina of
the eye by the lens in the front of the eye.
17.2 Mirrors, Lenses, and Images
 Objects are real
physical things that
give off or reflect light
rays.
 Images are “pictures” of
objects that are formed
in space where light
rays meet.
17.2 Mirrors, Lenses, and Images
 The most common image we see every day is our own
reflection in a mirror.
 The image in a mirror is called a virtual image because
the light rays do not actually come together.
 The virtual image in a flat
mirror is created by the
eye and brain.
17.2 Mirrors, Lenses, and Images
 Light rays that enter a converging lens parallel to its
axis bend to meet at a point called the focal point.
 The distance from the center of the lens to the focal
point is called the focal length.
 The optical axis usually goes through the center of the
lens.
17.2 The image formed by a lens
 A lens can form a virtual image just as a mirror does.
 Rays from the same point on an object are bent by the
lens so that they appear to come from a much larger
object.
17.2 The image formed by a lens
 A converging lens can also form a real image.
 In a real image, light rays from the object actually come
back together.
17.2 Drawing ray diagrams
 A ray diagram is the best way to understand what type
of image is formed by a lens, and whether the image is
magnified or inverted.
 These three rays follow the rules for how light rays are
bent by the lens:
1. A light ray passing through the center of the lens is
not deflected at all (A).
2. A light ray parallel to the axis passes through the far
focal point (B).
3. A light ray passing through the near focal point
emerges parallel to the axis (C).
17.3 Optical Systems
Key Question:
How are the properties of
images determine?
*Students read Section 17.3
AFTER Investigation 17.3
17.3 Optical Systems
 An optical system is a collection of mirrors, lenses,
prisms, or other optical elements that performs a useful
function with light.
 Characteristics of optical systems are:
— The location, type, and magnification of the image.
— The amount of light that is collected.
— The accuracy of the image in terms of sharpness,
color, and distortion.
— The ability to change the image, like a telephoto lens
on a camera.
— The ability to record the image on film or
electronically.
17.3 The sharpness of an image
 Defects in the image are called aberrations and
can come from several sources.
— Chromatic aberration is caused by dispersion,
when different colors focus at different
distances from the lens.
17.3 The sharpness of an image
— Spherical aberration causes a blurry image
because light rays farther from the axis focus to
a different point than rays near the axis.
17.3 The sharpness of an image
— Diffraction causes a point on an object to focus as
a series of concentric rings around a bright spot.
17.3 Thin lens formula
 The thin lens formula is a mathematical way to
do ray diagrams with algebra instead of drawing
lines on graph paper.
1 +1 =1
do di df
Object
distance
(cm)
Image distance
(cm)
focal
length (cm)
17.3 Use the thin lens formula
 Calculate the location of the image if the object is 6 cm
in front of a converging lens with a focal length of 4
cm.
17.3 Image relay
 A technique known as image relay is used to analyze
an optical system made of two or more lenses.
Application: The Telescope