Transcript Chapter 23 Ray Optics

Chapter 23. Ray Optics
Our everyday experience that
light travels in straight lines is the
basis of the ray model of light.
Ray optics apply to a variety of
situations, including mirrors,
lenses, and shiny spoons.
Chapter Goal: To understand and
apply the ray model of light.
In this chapter you will learn:
• Use the ray model of light
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Calculate angles of reflection and refraction
Understand the color and dispersion
Use ray tracing to analyze lens and mirror systems
Use refraction theory to calculate the properties of lens
systerm
Reading assignment
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The Ray Model of Light
Reflection
Refraction
Image Formation by Refraction
Color and Dispersion
Thin Lenses: Ray Tracing
Thin Lenses: Refraction Theory
Image Formation with Spherical Mirrors
Stop to think 23.1
Stop to think 23.2
Stop to think 23.3
Stop to think 23.4
Stop to think 23.5
Stop to think 23.6
Example 23.2
Example 23.4
Example 23.9
Example 23.11
Example 23.17
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Propagation of Light – Ray (Geometric) Optics
Main assumption:
 light travels in a straight-line path in a uniform
medium and
 changes its direction when it meets the surface of a
different medium or
 if the optical properties of the medium are nonuniform
The rays (directions of propagation) are straight
lines perpendicular to the wave fronts
The above assumption is valid only when
the size of the barrier (or the size of the
media) is much larger than the wavelength
of light
  d
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Stop to think 23.1
A long, thin light bulb illuminates a vertical aperture.
Which pattern of light do you see on a
viewing screen behind the aperture?
C
Reading quiz 1
A virtual image is
A virtual image is
A.the cause of optical illusions.
B.a point from which rays appear
to diverge.
C.an image that only seems to
exist.
D.the image that is left in space
after you remove a viewing
screen.
A.the cause of optical illusions.
B.a point from which rays
appear to diverge.
C.an image that only seems to
exist.
D.the image that is left in space
after you remove a viewing
screen.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Reading quiz 2
The focal length of a converging lens is
The focal length of a converging lens is
A.the distance at which an image is
formed.
B.the distance at which an object
must be placed to form an image.
C.the distance at which parallel
light rays are focused.
D.the distance from the front
surface to the back surface.
A.the distance at which an image is
formed.
B.the distance at which an object
must be placed to form an image.
C.the distance at which parallel
light rays are focused.
D.the distance from the front
surface to the back surface.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Reflection
The law of reflection states that
1. The incident ray and the reflected ray are in the
same plane normal to the surface, and
2. The angle of reflection equals the angle of
incidence: θr = θi
Reflection of Light
Specular reflection
(reflection from a
smooth surface) –
example: mirrors
Diffuse reflection
(reflection from a
rough surface)
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The Plane Mirror
Consider P, a source of rays which reflect from a
mirror. The reflected rays appear to emanate from
P', the same distance behind the mirror as P is in
front of the mirror. That is, s' = s.
Two plane mirrors form a right angle. How
many images of the ball can you see in the
mirrors?
A. 1
B. 2
C. 3
D. 4
C
Refraction
Snell’s law states that if a ray refracts between
medium 1 and medium 2, having indices of
refraction n1 an n2, the ray angles θ1 and θ2 in the
two media are related by
Notice that Snell’s law does not mention which is
the incident angle and which is the refracted
angle.
Refraction – Snell’s Law
• The incident ray, the refracted ray,
and the normal all lie on the same
plane
• The angle of refraction is related to
the angle of incidence as
sin  2 v2

sin 1 v1
– v1 is the speed of the light in the
first medium and v2 is its speed
in the second
Since v1 
sin  2 v2 c / n2 n1
c
c
and v2 
, we get
 
 , or n2 sin2  n1 sin1
n1
n2
sin 1 v1 c / n1 n2
Snell’s Law
index of refraction
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Color
Dispersion
Different colors are associated with light of different
wavelengths. The longest wavelengths are perceived as
red light and the shortest as violet light. Table 23.2 is a
brief summary of the visible spectrum of light.
The slight variation of index of refraction with
wavelength is known as dispersion. Shown is the
dispersion curves of two common glasses. Notice that n
is larger when the wavelength is shorter, thus violet
light refracts more than red light.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Refraction in a Prism
Since all the colors have different angles
of deviation, white light will spread out
into a spectrum
 Violet deviates the most
 Red deviates the least
 The remaining colors are in
between
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   1   1 ' 
when   
n=
sin 1

sin  2
min ,
sin(
2 2' 

min
2
sin


2

)
2

 min 
, 1   1 ' 

2
2
2
EXAMPLE 23.4 Measuring the index of
refraction
EXAMPLE 23.4 Measuring the index of
refraction
QUESTION:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 23.4 Measuring the
index of refraction
Total Internal Reflection
Total Internal Reflection: Application
Fiber Optics
• Plastic or glass rods are used
to “pipe” light from one
place to another
Total Internal Reflection
(  incidence   cr )
• Applications include:
– medical use of fiber optic
cables for diagnosis and
correction of medical
problems
– Telecommunications
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A triangular glass prism with an apex angle of Ф=60o has an
index of refraction n=1.5. What is the smallest angle of incidence
for which a light ray can emerge from the other side?
Thin Lenses: Ray Tracing
Thin Lenses: Ray Tracing
Thin Lenses: Ray Tracing
Lateral Magnification
The image can be either larger or smaller than the
object, depending on the location and focal length of the
lens. The lateral magnification m is defined as
1. A positive value of m indicates that the image is
upright relative to the object. A negative value of m
indicates that the image is inverted relative to the
object.
2. The absolute value of m gives the size ratio of the
image and object: h'/h = |m| .
Important Concepts
Applications