Which one of the following statements is correct about the magnification of an optical microscope?
- A.Magnification increases with the increase in focal length of eyepiece
- B.Magnification increases with the increase in focal length of objective
- C.Magnification does not depend upon the focal length of eyepiece
- D.Magnification decreases with the increase in focal length of eyepiece
Correct answer
Explanation
The correct answer is D, Magnification decreases with the increase in focal length of eyepiece. A compound microscope magnifies more when both its lenses have short focal lengths.
In a compound microscope the objective forms a real, enlarged image, and the eyepiece magnifies it further like a simple magnifier. The total magnification is roughly (L/fo) multiplied by (D/fe), where L is the tube length, D is the least distance of distinct vision, 25 cm, fo is the focal length of the objective and fe that of the eyepiece. Since both focal lengths sit in the denominator, a longer focal length of either lens gives a smaller magnification. That is why microscope lenses are small and strongly curved.
A is wrong because a longer eyepiece focal length lowers magnification. B is wrong for the same reason with the objective. C is wrong because the eyepiece's focal length appears directly in the formula.
Exam tip: microscope, both fo and fe small; telescope, fo large and fe small, magnification fo/fe.
Practice Questions
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- D.Sclerenchyma
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- B.the Left-Hand Thumb rule and it would be in the downward direction.
- C.the Right-Hand Thumb rule and it would be in the upward direction.
- D.the Left-Hand Thumb rule and it would be in the upward direction.
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- C.Sound is a mechanical wave and transverse in nature
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- B.Free electrons
- C.Both bound and free electrons
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- A.Copper
- B.Zinc
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- D.Manganese
Show answer
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