Classical Photography

1
The Geometric Optics of Image
FormationGraphical Ray Tracing
2
Using Refraction to Focus Light.
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Glass Lens in Air
Parallel Rays
n21.5
Focal point of lens
Optical Axis
Focal length of lens, f
3
Parallel rays come to focus at one pointon the
image plane.
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Glass Lens in Air
n21.5
Optical Axis
Parallel Rays different direction
Image Plane
Focal length of lens, f
4
A Chief Ray is a ray heading towardor away from
the center of the lens.
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Glass Lens in Air
Examples of Chief Rays
n21.5
Optical Axis
Focal length of lens, f
5
Thin Lens ApproximationChief Rays pass through
the lens without deviation.
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Glass Lens in Air
Examples of Chief Rays
n21.5
Optical Axis
Focal length of lens, f
6
We identify two very important rays
(A) Collimated Rays These are the rays that
are parallel to the optical axis. These
rays come to focus at the focal point. (B)
Chief Rays These are the rays that go through
the center of the lens on the optical
axis. These rays are un-deviated.
7
Light from the object passes through the
lens. Follow the Ray Tracing Rule
Thin Lens
Object
Optical Axis
f
8
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9
This is how a projector works.
Object is a slide
h
Light Source
f
f
h'
10
Magnification The ratio of the size of the
image to the size of the object.
Magnification lt 1 in this example, so the image
is smaller than the object.
Object
Optical Axis
f
11
Traditional Ray Tracing Terms
Focal lengths for a thin lens in air f f'
Object
(collimated ray)
(chief ray)
h
h'
f'
f
L
L'
Object distance and height (L, h)
Image distance and height (L, h)
12
Try some different object locations, L. We
observe 6 special cases.
Object distance L Image distance L'
h
h'
f
f
13
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14
Case (II) L between ? and 2f. As object moves to
the right, the image size increases.
h
h'
f
f
Image is real and inverted.
15
h
h'
f
f
16
Case (III) At L 2f, h h', and M 1.
h
h'
f
f
17
Case (IV) L between 2f and f, (a) the image is
still inverted, and (b) h' gt h, and M gt 1.
h' still increases as the object moves toward the
lens.
looks larger inverted
h
h'
f
f
18
For L between 2f and f, (a) the image is still
inverted, and (b) h' gt h, and M gt 1.
h' still increases as the object moves toward the
lens.
looks much larger inverted
h
f
f
h'
19
Case(V) L f. The rays are parallel.
They cross at infinity, so h' ? M.
This is the point of maximum confusion!
looks very confusing
h
f
f
20
Case (VI) L between f and the lens. The rays
diverge and look AS IF they come from an image
that (a) is erect and (b) enlarged, h'gth, m gt 1.
This is called a "virtual image".
We look through the lens, and it is a magnifying
glass!
h'
M gt 1
h
h
f
f
The rays diverge!!
21
This is how a magnifying glass works!
This is called a "virtual image".
We look through the lens, and it is a magnifying
glass!
h'
M gt 1
h
h
f
f
The rays diverge!!