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Simple microscopes

  1. Aug 18, 2013 #1
    I performed a simple experiment to find the focal length of a magnifying glass. Taking a white sheet of paper as a screen and a building as object at infinity, i got a clear image at 12.5 cm, which is approximately the focal length of the lens. Plugging this into the formula for magnifying power of a simple microscope, we get the maximum and minimum powers as 3 and 2. However the carton says the magnifying glass is 5x. Plugging into the formula we get focal length 5 cm, and when a tried it, i couldnt get an image at all. So what is wrong? The formula i used for power of simple microscope was 25 divided by f for minimum power and 1 plus 25 divided by f for maximum power.
  2. jcsd
  3. Aug 18, 2013 #2


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    I suspect that either the manufacturer accidentally used the wrong carton for that magnifier, or else the people who wrote the text for the carton used some "creative license." :wink:

    [added] Or maybe the person who tested the magnifier for them is an old geezer with a "near point" viewing distance greater than the 25 cm that most textbooks use as a standard.
    Last edited: Aug 18, 2013
  4. Aug 18, 2013 #3
    Im 14 years old! Initially i thought i mustve made a mistake, but i checked many times. Also the carton has no manufacturers name, so maybe they pulled a fast one :p
  5. Aug 18, 2013 #4


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    If you're only 14, your near point distance is probably half (or maybe even less) of the 25 cm standard. When you hold one end of a ruler close to your eye, how close along the scale can you focus?

    When I started teaching, I could focus down to about 15 cm with my glasses on, or less than 10 cm with my glasses off (I'm very nearsighted). Now, I have to wear bifocals. My near point has moved outward and merged with my far point: around 15 cm in one eye and 20 cm in the other.
  6. Aug 19, 2013 #5


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    As people get older, their arms just get shorter and they can't hold a newspaper far enough away to read it. :wink:
  7. Aug 19, 2013 #6


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    Well, 3 + 2 = 5, isn't it? :-p

    To be fair, it is kinda bad from the manufacturer. But I don't think many parents will be returning their purchase because of this marketing error.

    Actually I never did get to see anything interesting through such a basic microscope anyway - probably your little experiment explains why :)
  8. Aug 19, 2013 #7
    @jtbell: I don't think my near point should have anything to do with the experiment. When the image is formed at the focus from an object at infinity, a child or an old person should have no trouble seeing it from a comfortable distance. The focal length is constant. Maybe if a person is near blind, he won't be able to see the image on the white paper but nevertheless, technically the image formed when the paper is a focal length away will be less blurred that the other positions :P
    Once we get the focal length we just put it in the formula to get the magnification. Here's where the near point of the observer comes in. The magnifying power derived from the formula will be for an average adult, and I suspect that's what the manufacturers are supposed to put. Even if the magnification is less for me because D for me is less than 25, that is an observed effect unique to me. The magnification should still be standard(for an adult).

    @sophiecentaur- :)

    @CompuChip- Unless they are physicists ;)
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