Download MIRRORS & LENSES

Chapter 18: Refraction and Lenses
Pre-Class for Tuesday 4-10-12
In terms of light transmission, what is the
difference between mirrors and lenses?
• Definition: bending of light as
it passes from one material to
another
• Cause: light travels slower
through more optically dense
materials
• The optical density is measured
by the index of refraction (n),
which is the ratio of the speed
of light in a vacuum (c) to the
speed of light in that material
(v)
n=c/v
• Describes how light
behaves when passing
from one material to
another
Incident
• Light passes through an
ray
object, but is deflected
off its original line
Normal
Refracted • Amount of refraction is
based on the object’s
ray
optical density and
incident angle
n1sinq1 = n2sinq2
Total Internal Reflection
• Light attempts to pass from one material to
another
• Light must be traveling from a more dense
to a less dense material
• If the incident angle is larger than the
critical angle (qc), then light is reflected
• Law of Reflection applies (qi=qr)
sin qc = n2/n1
Pre-Class Wednesday 4-11-12
What causes a
rainbow?
3 TYPES OF LENSES
• PLANE
– flat lens (window)
– always gives a real image
• CONVEX
– curved towards the object (symmetric on both sides)
– gives virtual or real images
• CONCAVE
– curved away from object (symmetric on both sides)
– gives virtual or real images
LENS EQUATION
1/f = 1/di + 1/do
f : focal length
di : distance to image from lens
do : distance to object from lens
MAGNIFICATION
M = hi / ho = - di / do
M : magnification
hi : height of image
ho : height of object
di : distance from image to lens
do : distance from object to lens
SIGNS
– If di is negative, virtual image
– If di is positive, real image
– If M is negative, inverted image
– If M is positive, upright image
– rule of thumb: real images are what we
expect, so they are positive (light passes
through lens)
– remember: just because M is negative does
not mean it is smaller, the sign is for
orientation only
Ray Diagrams for Lenses
1) // OA, thru f1
2) Thru f2, then // OA
3) Thru center of lens
Note: use refraction line and not actual lens surface.
HW CH 18
67. 1.33; water
69. 25.4o; 28.9o
70. 1.24x108 m/s
71. 24.4o
73. 53o, shallower
75. 1.96x108 m/s;
1.99x108 m/s
78. 28o; 32o; 53o
82. 39.3 cm
83. 10 cm
84. 14 cm
86. -1.5 cm
92. 35 mm, do is infinity
93. 60 mm, -5, -20 mm, -10
94. 21 cm, -0.494 cm,
inverted; -324 cm, -40 cm,
inverted; -4
100. 0.19 mm
Pre-Class Thursday 4-12-12
Which color of light travels the
slowest through a material?
a) Red
b) Green
c) Yellow
d) Violet
Ch19: Interference
and Diffraction
• Diffraction is the
scattering of light
from a surface.
• Incoherent light:
light
that travels
Sunlight
or lightin
all directions with
circular bulb
wavefronts
• Coherent light:
lightorthat
travels inbeam
Laser
collimated
one direction with
linear wavefronts
Practice Problems
Complete the following four:
p487 #1-4
Choose three of following:
p496 #15-17
p497 #20-23
One piece of paper per group.
Be sure everyone’s name is on it.
Lenses Activity
• For each person,
find the position for
each lens that
produces a sharp,
clear image.
• Repeat for 2 lenses
together
Your
name
Type of
lens
Lens to
eye
M
Upright or
inverted
both
Lens to
lens
M
Upright or
inverted
Pre-Class Wednesday 4-15-09
What type of lens is
in your eye?
Hint: there are only three
possibilities.
Pre-Class Thursday 4-16-09
What is the smallest
value that the index
of refraction can be?
LENS LAB
•
Materials:
–
•
Objective:
–
•
meter stick, optical bench kit, lenses
Find the focal length for each lens
Procedures:
1) Fill-out the chart on the next slide
2) Draw ray diagrams for each different lens
3) Repeat step 1 as time allows for a different lens
FILL-IN CHART
LENS
Convex 1
Convex 2
Convex 3
Concave 1
Concave 2
Concave 3
di
do
f
NOTE: Be sure image is sharp and clear. Look for it on
the correct side (real or virtual image).