The Compound Microscope
Of course, we can use more than one external lens to create the enlarged image for your eye. For example, a compound microscope uses two lenses (and so your eye’s lens is the third in that case).
The first lens (the objective ) is positioned so the object is just outside its focal point. This produces an enlarged (inverted) real image.
Then the second lens (the eyepiece ) is positioned so that this image is located at its focal point. That is, the eyepiece acts as magnifying glass viewing the image formed by the objective lens.
The result is a large virtual image.
The angular magnification is: which reduces to:
M =  ’/  ≈ ( h i2
/ d i2
M ≈ – NP · ( L – f e
)/( f e
· f
)/( h o o
)
/ NP )
Again, NP is your eye’s near point. L is the distance between the two lenses, with L > f e
+ f o
. And notice: L = d i1
+ f e
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Figure 24.13B
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The Telescope
The same angular magnification strategy works no matter whether the object’s size is small or its distance is huge; either case produces a very tiny viewing angle,  .
We’ve seen how a microscope can help the first case. A telescope helps the latter:
The first lens, the objective , forms a real, inverted image at its focal point. (Why there?) The second lens, the eyepiece , is positioned so that this image is located at its focal point. (Why there?)
Again, the result is a virtual image that subtends a large angle. In this case, M ≈ – f o
/ f e
, and L (the distance from the objective to the eyepiece) is given by L ≈ f e
+ f o
.
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Figure 24.14
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A Summary of Angular Magnification
Definition : M =  ’ / 
The value of M as calculated for common instruments :
Magnifier, “comfort” (standard) viewing: M ≈ NP / f
Magnifier, maximum (strained) viewing: M ≈ NP / f + 1
Microscope, “comfort” (standard) viewing: M ≈ – NP ( L – f e
)/( f o
· f e
)
Telescope, “comfort” (standard) viewing: M ≈ – f o
/ f e
Note: Some of these instrument “ratings” are different (more general) than those presented in the textbook . For a more detailed summary and discussion of the differences (and why we’ll be using the above), be sure to read After Class 18 .
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Problem 11a.
HW5-6 Tips/Hints
Problem 11b.
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