how can you change the power of a single lens microscope
You can't, if you mean a microscope with a single, tiny, spherical lens. A sphere allows you to observe an object closer to your eye than 10 inches (which is conidered the closest, normal viewing distance for the human eye). [ A sphere one inch in diameter allows you to observe an object just as close (or one inch). Since this is ten times closer to your eye, the object appears with a
magnification of 10 power.
If you want to see an object (through a single lens) with even more magnification, you need a smaller sphere. It's possible to produce spheres with diameters less than 1/10th of an inch. A single spherical lens that small creates maginifications of 100 power.
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