NCERT Solutions for Class 9th Science Chapter 2 Activity 2.1: Let us estimate the size of a cell — How to Study Cells?
Book page 10 Updated on2026-09-08
Q1.
Estimate the real size of the cell using the formula: Estimated size of the onion peel cell = Diameter of the visible field in micrometre / Number of cells along the diameter
Answer
Take the book's own readings: field diameter 5 mm, and 25 onion peel cells counted along that diameter.
Step 1 — convert the field diameter to micrometres.
1 mm = 1000 µm
d = 5 mm = 5 × 1000 µm = 5000 µm
Step 2 — apply the formula.
estimated size of one cell = d ÷ n
= 5000 µm ÷ 25
= 200 µm = 0.2 mm
The field of view is used as a ruler: its known real diameter is shared out among the cells that fit across it.
Why it happens: the method works because the cells and the field of view are magnified by exactly the same factor. Dividing one by the other cancels the magnification completely, so the answer is a real size even though it was obtained from a magnified image. That is also why the answer is only an estimate — it assumes the cells lie edge to edge with no gaps and that the count of 25 is exact.
Check it yourself: if 40 cells had fitted instead of 25, each cell would be 5000 µm ÷ 40 = 125 µm. Smaller cells, more of them — the two numbers are inversely related.
Q2.
If the estimated size of an onion peel cell is 200 µm, how much does a light microscope magnify this cell?
Answer
The total magnification is the product of the two lenses.
total magnification = magnification of eyepiece × magnification of objective
= 10X × 10X
= 100X
apparent size = real size × magnification
= 200 µm × 100
= 20 000 µm = 20 mm = 2 cm
So the cell that is really only 0.2 mm wide appears about 2 cm wide — roughly the width of your thumbnail.
Why it happens: the objective forms a real, enlarged image inside the body tube; the eyepiece then treats that image as its object and enlarges it again. Because the second lens works on the output of the first, the two factors multiply rather than add. Switching to the 40X objective with the same eyepiece would give 10 × 40 = 400X, and the same cell would appear 200 µm × 400 = 80 000 µm = 8 cm across.
Tip: higher magnification is not automatically better. At 400X the field of view is much smaller and the image is dimmer, so you see fine detail of a very small patch. Choose the objective that suits the question.