Class 19 - Physics at Oregon State University

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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

2/16/15 Oregon State University PH 212, Class #19 1

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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