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Chapter 18: Refraction and Lenses
Pre-Class for Tuesday 4-10-12In 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 / vn = c / v
• Describes how light behaves when passing from one material to another
• Light passes through an object, but is deflected off its original line
• Amount of refraction is based on the object’s optical density and incident angle
Incident ray
Normal
Refracted ray
n1sin1 = n2sin2
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 (c), then light is reflected
• Law of Reflection applies (i=r)
sin c = 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 EQUATIONLENS EQUATION
1/f = 1/di + 1/do
f : focal length
di : distance to image from lens
do : distance to object from lens
MAGNIFICATIONMAGNIFICATION
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
S I G N SS I G N S– 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
This Way
No, this way
F=m a
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 18HW 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: Ch19: Interference and Interference and
DiffractionDiffraction• Diffraction is the
scattering of light from a surface.
• Incoherent light: light that travels in all directions with circular wavefronts
• Coherent light: light that travels in one direction with linear wavefronts
Sunlight or light
bulbLaser or
collimated beam
Practice Problems
Complete the following four:p487 #1-4
Choose three of following:p496 #15-17p497 #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:– meter stick, optical bench kit, lenses
• Objective:– 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 di do fConvex 1Convex 2Convex 3Concave 1Concave 2Concave 3
NOTE: Be sure image is sharp and clear. Look for it on the correct side (real or virtual image).