The Blurry Bacteria
Sam puts a smear of yogurt under the school microscope at 400×. The bacteria in it show up only as tiny grey specks.
Would more magnification show what the bacteria really look like?
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Science · Grade 7 Science
Chapter 1: Cells: The Unit of Life
Magnification is not the same as resolution.
Sam puts a smear of yogurt under the school microscope at 400×. The bacteria in it show up only as tiny grey specks.
Would more magnification show what the bacteria really look like?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children treat the magnification number as the measure of a microscope, because bigger feels like better. The test slide turns detail into a count, and the screen zoom multiplies the size by 4 while the count stays at 4. Ask: what changed between 1,000× and 4,000×, and what did not? The idea to reach is that magnification sets size and resolution sets detail.
Why the stronger objectives also resolve better — Resolution depends on the wavelength of light and on the numerical aperture of the objective, roughly how wide a cone of light it collects. Higher-power objectives are made with larger apertures, and the 100× oil-immersion lens reaches about 0.0002 mm, so detail rises with objective power up to about 1,000×. Beyond that, magnification is "empty". Technique still matters: start at low power, focus with the coarse knob, then switch up and use fine focus only.
About the numbers — Visible light spans roughly 0.0004 to 0.0007 mm in wavelength; the best light microscopes resolve about half of that, near 0.0002 mm. Lactobacillus rods in yogurt are about 0.5–1 micrometer wide and a few micrometers long. The test-slide counts are an illustration consistent with typical 10×, 40× and 100× objectives, not a published measurement.