Transcript Geometric Optics - Grade 12 Physics

CHAPTER 14
REFRACTION
Ms. Hanan
14-2 Thin Lenses
Objectives:
• Use ray diagram to find the position of an image
produced by a converging or diverging lens, and
identify the image as real or virtual.
• Solve problems using the thin-lens equation.
• Calculate the magnification of lenses.
• Describe the positioning of lenses in compound
microscopes and refracting telescopes.
14-2 Thin Lenses
Vocabulary:
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Converging lens
Convex lens
Diverging lens
Concave lens
Ray diagram
Focal point
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Focal length
Centre of the lens
Real image
Virtual image
Magnification
14-2 Thin Lenses
The first telescope, designed and built by Galileo, used lenses to focus light from
faraway objects, into Galileo’s eye. His telescope consisted of a concave lens and a
convex lens.
light from
far away
object
convex
lens
concave
lens
Light rays are always refracted (bent) towards the thickest part of the lens.
Concave (Diverging) Lenses
Concave lenses are thin in the middle and make
light rays diverge (spread out).
•
F
Principal axis
If the rays of light are traced back (dotted sight lines),
they all intersect at the focal point (F) behind the lens.
Concave (Diverging) Lenses
•
F
Principal axis
Light
Therays
light that
rayscome
behave
in parallel
the same
to the
wayoptical
if we ignore
axis diverge
the thickness
from the
offocal
the lens.
point.
Concave (Diverging) Lenses
•
F
Principal axis
Light rays that come in parallel to the optical axis still diverge from the focal point.
Concave (diverging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
Concave (diverging) Lens
(example)
•
F
principal axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
Concave (Diverging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
Concave (Diverging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts from the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
A virtual image forms where the sight lines converge.
Your Turn
(Concave (Diverging) Lens)
object
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F
Principal axis
concave lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Your Turn
(Concave (Diverging) Lens)
object
•
Fimage
Principal axis
concave lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Convex (Converging) Lenses
Convex lenses are thicker in the middle and focus light rays to a focal point in front of
the lens.
The focal length of the lens is the distance between the center of the lens and the
point where the light rays are focused.
Convex (converging) Lenses
•
F
Principal axis
Convex (converging) Lenses
•
F
Principal axis
Light rays that come in parallel to the optical axis converge at the focal point.
Convex (converging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
Convex (converging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
Convex (converging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
Convex (converging) Lens
(example)
•
F
Principal axis
The first ray comes in parallel to the optical axis and refracts through the focal point.
The second ray goes straight through the center of the lens.
The light rays don’t converge, but the sight lines do.
A virtual image forms where the sight lines converge.
Your Turn
(Convex (converging) Lens)
Principal axis
•
F
object
convex lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Your Turn
(Convex (converging) Lens)
•
F
object
Principal axis
image
convex lens
• Note: lenses are thin enough that you just draw a line to represent the lens.
• Locate the image of the arrow.
Rules for Drawing Reference Rays
Ray
From object to lens
From converging lens to image
Parallel ray
(P ray)
Parallel to principal axis
Passes through focal point, F
Central ray
(M ray)
To the center of the lens
From the center of the lens
Focal ray
(F ray)
Passes through focal point, F
Parallel to principal axis
Ray
From object to lens
From diverging lens to image
Parallel ray
(P ray)
Parallel to principal axis
Directed away from focal point, F
Central ray
(M ray)
To the center of the lens
From the center of the lens
Focal ray
(F ray)
Proceeding toward back focal
point, F
Parallel to principal axis
Ray Tracing for Lenses
These diagrams show the principal rays for both types of lenses:
Lens & Mirror Equation
1 1 1
 
f
p q
ƒ = focal length
p = object distance
q = image distance
f is negative for diverging mirrors and lenses
di is negative when the image is behind the lens or mirror
Magnification Equation
h q
M 
h
p
'
M = magnification
h’= image height
h = object height
If height is negative the image is upside down
if the magnification is negative
the image is inverted (upside down)
The Thin-Lens Equation
Sign conventions for thin lenses:
M
M
q
q
p
p
Assignments
• Class-work:
Practice B, page 501, odd questions.
• Homework:
Practice B, page 501, even questions.
Homework due next class
Eyeglasses and Contact Lenses
Vocabulary:
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Myopia
Short Sightedness
Hyperopia
Far sightedness
Objective Lense
Eye Piece
• Compound Microscope
• Refracting Telescope
Eyeglasses and Contact Lenses
Leads to the
occipital
cortex at the
posterior
(back) of the
brain
Anatomy of the Human Eye
Normal Vision
The process in which the lens changes its focal length to focus on
objects at different distances is called accommodation
Myopia, Hyperopia
If the incoming light from a far away
object focuses before it gets to the back
of the eye, that eye’s refractive error is
called “myopia” (nearsightedness).
If incoming light from something far away
has not focused by the time it reaches the
back of the eye, that eye’s refractive error
is “hyperopia” (farsightedness).
Myopia - Nearsightedness
Hyperopia - Farsightedness
Combination of Thin Lenses
In lens combinations, the image formed by the first lens becomes the
object for the second lens (this is where object distances may be negative).
The Compound Microscope
• In the basic compound microscope, the object to be
magnified is placed under the lower lens (objective
lens) with a focal length of less than 1 cm, and the
magnified image is viewed through the upper lens
(eyepiece lens) with a focal length of few centimeters.
• The magnification of the image can be calculated by
multiplying the magnifying power of the objective lens
times the magnifying power of the eyepiece lens.
• The microscope is composed of a mechanical system
which supports the microscope, and an optical system
which illuminates the object under investigation and
passes light through a series of lens to form an image
of the specimen.
The Compound Microscope
The principle of the compound microscope. The passage of light through two
lenses forms the virtual image of the object seen by the eye.
The Refracting Telescope
Assignments
• Homework:
Section review, page 505, questions: 1, 2, 3, 4, 5,
and 6.
Homework due next class