The medium-power objective lens in a laboratory microscope has afocal length f o

Question

The medium-power objective lens in a laboratory microscope has afocal length fobjective = 4.80 mm. Use 25 cm for the near-point. (a) If this lens produces a lateralmagnification of -39.0, what is its "working distance"; that is,what is the distance from the object to the objectivelens?
______? mm
(b) What is the focal length of an eyepiece lens that will providean overall magnification of 119?(Include the sign.)
_______? cm (a) If this lens produces a lateralmagnification of -39.0, what is its "working distance"; that is,what is the distance from the object to the objectivelens?
______? mm
(b) What is the focal length of an eyepiece lens that will providean overall magnification of 119?(Include the sign.)
_______? cm

Explanation / Answer

1 / o + 1 / i = 1 / f f = o i / (o + i) m = -i / o = -39        i =39 o f = 39 o2 / 40 o o = 40 / 39 f = 40 * 4.8 mm = 4.92 mm    (justoutside the focal point of the objective) M = (25 / f) + 1    where M is themagnification of a simple magnifier with the image at the nearpoint 119 = M * 39 = 25 / f + 1 M = 119 / 39 = 3.05 25 / f + 1 = 3.05 f = 25 / 2.05 = +12.2 cm (Can be a little confusing as the overall magnification of amicroscope is negative - inverted image)

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