Measurements, Microscopes & Scale — Practice · Grade 7 Science

Chapter: Scientific Inquiry & Diversity of Life

What this practice covers

By the end of Measurements, Microscopes & Scale you should be able to:

8 questions, written answer, multiple choice and numeric, each with a worked explanation.

Vocabulary used: magnification, resolution, micrometer, scale bar, calibration.

Problem 1

Why is “a cell is 20 long” incomplete?
Answer and explanation

It omits the unit and measurement method. A cell might be 20 micrometers long, but the number alone cannot establish its size.

Problem 2

Does changing from 40× to 100× make the real cell larger?
Answer and explanation

No. It enlarges the image and changes the viewed area; the actual cell size is unchanged.

Problem 3

A cell image is half the length of a 100-micrometer scale bar. What size is supported?
Answer and explanation

Its estimated length is 50 micrometers, assuming the bar belongs to that image and both are measured in the same direction.

Problem 4

Why does not seeing ribosomes fail to show that a cell lacks them?
Answer and explanation

They are too small for many classroom microscopes to resolve. Instrument limits can explain the missing visible detail.

Problem 5

Illustrative image record: a 4 cm scale bar represents 200 micrometers; the cell image spans 1.2 cm. A microscope setting lists a 10× eyepiece and a 40× objective. Calculate estimated cell length and total magnification. Explain why these are different quantities and state a calibration assumption.
Answer and explanation

The cell length is approximately 60 micrometers, and the stated total magnification is 400×. Length describes the specimen estimate; magnification describes image enlargement. Both assume the provided record is correct, and the scale calculation assumes uniform resizing of the cell and bar. Magnification alone does not supply the real cell length.

Find the ratio 1.2/4=0.3.

Multiply 0.3 by 200 micrometers.

Multiply lens factors 10×40.

Check that image and scale bar were resized together.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

A drawing is enlarged while its copied scale bar is left unchanged. Can that bar still calibrate the enlarged cell?
Answer and explanation

No. The bar no longer has the same image-to-specimen relationship. Resize it with the image or provide a new valid calibration.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

Which improves the ability to separate nearby details?
  1. A larger caption
  2. Resolution
  3. Changing the real cell size by focusing
  4. Removing units
Answer and explanation

Resolution

Resolution concerns distinguishable detail, whereas magnification concerns enlargement.

Problem 8

A 10× eyepiece and 25× objective give what total magnification?
Answer and explanation

250 ×

10×25=250×.

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