Review Question:
Lenses
Which way will the light beam refract?
Ray refracted
towards normal
Ray refracted
away from
normal
air
glass
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2
Lens Operation:
Convex Lens
A lens bends light through refraction.
You can think of a lens as many pieces of glass
assembled together:
object
Principle axis
Focal point
2f
Focal Pt.
Focal point
Image
Convex Lens viewed as several pieces.
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Object past 2 focal points away. 2 or more rays
converge to show real, inverted, and smaller
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image.
Convex Lens
Convex Lens
image
object
object
Principle axis
Principle axis
2f
Focal point
2f
Focal point
Focal point
Focal point
Object between f and lens
Image
Object between 2f and f gives an inverted, real,
larger image.
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The rays don’t seem to actually converge.
But if they are traced backwards to their apparent
origin, a virtual, upright, and larger image is
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formed!!
Mirror / Lens Equation:
Questions:
Q: Where must an object be placed with a convex
lens so that it acts as a magnifying glass?
A: Inside the focal point
Q: Where must an object be placed with a convex
lens so that it projects a larger image onto a screen
(slide projector)?
If an object is placed 12 cm from a convex lens
with a focal length of 10 cm, where does the
image appear?
1 1 1
 
f do d i
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5 1


60 60 di
A: Between 2f and f
1
1 1
 
10 12 di
1
1

60 di
6
5 1


60 60 di
di  60 cm
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Mirror / Lens Equation:
Lens Combinations
If an object is placed 10 cm from a convex lens
and an image is clearly projected onto the Board
100 cm away, what is the lens’ focal length?
1 1 1
 
f do d i
1 1
1
 
f 10 100
11 1

100 di
100
 di
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A refracting telescope uses one large objective convex
lens in the front.
1 10
1


f 100 100
Another small convex lens is used as an eyepiece that
you look through.
di  9.1 cm
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Refracting Telescope
Light from a distant planet goes through the front
convex lens and forms a small, real image.
In reality, many optical devices use more than
one lens or mirror.
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Optical Devices:
Obviously, many devices employ the
principles learned today:
Telescope tube
The image is inside the focal point of the second
convex lens, the eyepiece.
The image acts as an object that then forms a much11
larger virtual image that the person sees.
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