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).