Why Physics?

1
Now consider refraction through this piece of
glass
focal point
optic axis
converging light
This is called a Double Convex Lens
2
Converging Lenses (convex) Diverging
Lenses (concave)
3
Diverging Lens Where is the focal point for these
4 incoming rays?
Converging Lens The focal point is visible
because real rays go through it.
4

• Optical Ray Diagram a line drawing depicting a
  small number of key light rays. For a lens, an
  optical ray diagram should include
• Parallel Ray. A ray parallel to the optic axis
  which passes through the object the focal
  point.
• Focal Ray. A ray that passes through both the
  focal point and the object.
• Chief Ray. A ray that passes through both the
  center of the lens and the object.

These three rays intersect at the image.
Note we dont use reflected rays in lens
analysis.
5
The Diverging Lens
real part
parallel ray
f
f
?
?
Virtual part where the refracted ray appears to
come from.
virtual part
6
The Diverging Lens
focal ray
real part
virtual part
?
?
Virtual part where the refracted ray appears to
come from.
7
The Diverging Lens
chief ray
?
?
The chief ray has no virtual part.
8
Put all three rays together
?
?
The three refracted rays have no intersection.
9
Put all three rays together
object
?
?
focal point
upright image
There is an intersection of the virtual parts.
Need virtual parts to find the image? ? Virtual
image.
10
The Thin Lens Equation
Parallel ray
a converging lens
Chief ray
Focal ray
?
?
f
so
si
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The Thin Lens Equation
Parallel ray
a converging lens
Chief ray
Focal ray
?
?
Positive side for object distance Negative side
for image distance focal length (e.g. diverging
lens)
Positive side for image distance focal length
12
The Thin Lens Equation
Strength or Power of lens
Note our book uses P. Other books use S.
f, so and di all must have the same length units.
Units of P usually in m-1 or rather Diopters.
13
Why is 1/f called the lens power?
(or sometimes strength)
focal point at ?
approaches
14
Why is 1/f called the lens power?
(or sometimes strength)
focal point very close to the lens
approaches
15
Example 1 A lens focuses light from an object
2.75m away as an image 0.483m on the other side
of the lens. What are the focal length, lens
type and image type?
The lens is converging because ? fgt0
Converging lens fgt0 Diverging lens flt0
What is the image type?
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The image is real (sigt0). Is it inverted or
upright?
2.75 m
0.483m
6 cm 1m
0.411m
?
?
Ray diagram shows the image is ? Real ?
Inverted
True whenever the object is outside the focal
point of a converging lens.
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The ray diagram also shows the image is small.
positive side for di and f
negative side for di and f
ho
?
?
hi
Minus sign indicates that real images are always
inverted.
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• How to do lens problems graphically
• Use a full sheet Landscape.
• Sketch the lens on the optic axis.
• Sketch the objects correctly positioned.
• Show a scale. You might wish to show a
  different scale for vertical and horizontal
  lengths
• Sketch two principle rays per object and find
  the image.
• 6. Refraction occurs on the vertical center-line.

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Example 2 How far from a converging lens with a
focal length of 25 cm should an object be placed
to produce a real image which is the same size as
the object?
(Minus because all real images are inverted.)
20
How to make a magnifying glass
What kind of lens has an upright image with mgt1?

• si negative
• si gt so
• image is farther from the lens
  than the object

si lt0
?
?
f
Place the object within the focal length of a
converging lens.