Transcript Document

Chapter 17 Optics
 17.1 Reflection and Refraction
 17.2 Mirrors, Lenses, and Images
 17.3 Optical Systems
Chapter 17 Objectives

Describe the functions of convex and concave lenses, a prism, and a
flat mirror.

Describe how light rays form an image.

Calculate the angles of reflection and refraction for a single light ray.

Draw the ray diagram for a lens and a mirror showing the object and
image.

Explain how a fiber-optic circuit acts like a pipe for light.

Describe the difference between a real image and a virtual image
and give an example of each.
Chapter 17 Vocabulary Terms














angle of refraction
chromatic
aberration
converging lens
critical angle
diffuse reflection
dispersion
diverging lens
eyepiece
fiber optics
focal length
focal plane
focal point
focus














geometric optics
image
image relay
incident ray
index of refraction
law of reflection
lens
magnification
magnifying glass
mirror
normal line
object
objective
optical axis













optics
prism
ray diagram
real image
reflected ray
refracting
telescope
Snell’s law
specular reflection
spherical
aberration
telescope
thin lens formula
total
internal reflection
Inv 17.1 Reflection and Refraction
 Investigation Key Question:
How do we describe the reflection
and refraction of light?
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.
Law of reflection
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.
1.
You are asked for a ray diagram
and the angle of reflection.
2.
You are given the angle of incidence.
3.
Use the law of reflection which states the angle of
reflection equals the angle of incidence.
4.
Sketch a ray diagram showing the angle of reflection is
30o.
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 i = nr sin r
Index of
refraction of
incident
material
Index of
refraction of
refractive
material
17.1 Relection and the critical angle
 The angle of incidence at which light begins
reflecting back into a refractive material is
called the critical angle.
 The critical angle depends on the index of
refraction.
Calculating 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.
1.
You are asked for the angle of refraction.
2.
You are given the angle of incidence, and materials.
3.
Use Snell’s law: ni sin i = nr sin r
4.
Solve sin r = 1.00 sin (30o) ÷ 1.33
sin r = 0.376
r = sin-1 (0.376) = 22o
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.
Chapter 17 Optics
 17.1 Reflection and Refraction
 17.2 Mirrors, Lenses, and Images
 17.3 Optical Systems
Inv 17.2 Mirrors, Lenses, and Images
Investigation Key Question:
How does a lens or mirror
form an image?
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).
17.2 Characteristics of images
 A real image is formed by the actual intersection
of light rays from the object.
 Real images can be projected on a surface.
17.2 Characteristics of images
 A converging lens acts as a magnifying glass
when the object is closer to the lens than one
focal length.
17.2 Characteristics of images
 A diverging lens is thicker around the edges and
thinner in the center.
 The image formed by a diverging lens is virtual
and right side up.
17.2 Characteristics of images
 The
magnification
varies for single
lenses- it
depends on the
distance of the
object from the
lens.
Chapter 17 Optics
 17.1 Reflection and Refraction
 17.2 Mirrors, Lenses, and Images
 17.3 Optical Systems
Inv 17.3 Optical Systems
Investigation Key Question:
How are the properties of
images determine?
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 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)
Locating an image
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.
1.
You are asked for the image distance.
2.
You are given focal length and object distance.
3.
Use thin lens formula 1 +
do
4.
Solve for 1
di
= 1 -1
4
1 =
1
di
df
= 3 - 2 = 1
6
12 12 12
di = 12 cm
17.3 Changing the size of an image
 A technique known as image relay is used to
analyze an optical system made of two or more
lenses.
17.3 Recording images
 There are two basic
techniques for recording
images.
 Film records an image by
using special inks that
respond to light.
 A digital camera uses a
tiny sensor called a CCD.
17.3 Recording images
 There are separate light sensors for
red light, blue light, and green light.
 A color image is recorded as a table
of numbers.
 Each point on the image has three
numbers corresponding to the
amount of red light, blue light, and
green light.
17.3 Recording images
 The resolution of a digital camera
is the number of points, called
pixels, that can be recorded by
the CCD.
 A 2-megapixel camera stores 2
million pixels per image.
 Since each pixel is three numbers,
a 2-megapixel image requires 6
million stored numbers.
17.3 The Telescope
 When people think of a telescope, most of them
think of a refracting telescope.
 An astronomical refracting telescope is
constructed of two converging lenses with
different focal lengths.
 The lens with the longest focal length is called
the objective and the shorter-focal-length lens is
the eyepiece.
17.3 The Telescope
 The image from this refracting telescope is
inverted which is usually fine for looking at
objects in space.
17.3 The Telescope
 The design of the terrestrial telescope sets the
lenses a distance apart equal to the sum of their
focal lengths.
 This design aids viewing animals or birds right
side up.
Newtonian Reflecting Telescope
 The most successful reflectingtelescope design is called a Newtonian
telescope, after Sir Isaac Newton, who
designed and built the first one.
 The larger the diameter of the objective
of a telescope, the more light it can
gather to form an image.
 The Hubble telescope is unique
because it orbits well above Earth’s
atmosphere so there is no distortion
from atmospheric refraction.