2/15/16 Oregon State University PH 212, Class #19 1 The Compound Microscope Of course, we can use more than one external lens to create the en-larged 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: M = ’/

2/15/16Oregon State University PH 212, Class #191 The Compound Microscope Of course, we can use more than one external lens to create the en- larged image

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2/15/16Oregon State University PH 212, Class #193 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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2/15/16 Oregon State University PH 212, Class #19 1

The Compound Microscope

Of course, we can use more than one external lens to create the en-larged 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: M = ’/ ≈ (hi2/di2)/(ho/NP)which reduces to: M ≈ –NP·(L – fe)/(fe·fo)

Again, NP is your eye’s near point. L is the distance between the two lenses, with L > fe + fo. And notice: L = di1 + fe

Page 2: 2/15/16Oregon State University PH 212, Class #191 The Compound Microscope Of course, we can use more than one external lens to create the en- larged image

Figure 24.13B

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

Page 3: 2/15/16Oregon State University PH 212, Class #191 The Compound Microscope Of course, we can use more than one external lens to create the en- larged image

2/15/16 Oregon State University PH 212, Class #19 3

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 ≈ –fo/fe, and L (the distance from the objective to the eyepiece) is given by L ≈ fe + fo.

Page 4: 2/15/16Oregon State University PH 212, Class #191 The Compound Microscope Of course, we can use more than one external lens to create the en- larged image

Figure 24.14

2/15/16 4Oregon State University PH 212, Class #19

Page 5: 2/15/16Oregon State University PH 212, Class #191 The Compound Microscope Of course, we can use more than one external lens to create the en- larged image

2/15/16 Oregon State University PH 212, Class #19 5

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 – fe)/(fo·fe)

Telescope, “comfort” (standard) viewing: M ≈ –fo/fe

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.