Cells are extremely small and cannot be seen with the naked eye. In this activity, you will learn how to estimate the size
of an onion peel cell using a compound microscope
and some simple mathematics.
Aim
To estimate the size of an onion peel cell using the microscope's field of view.
Materials Required
Transparent plastic ruler (with mm markings)
Compound microscope
Prepared onion peel slide
Lens paper (for cleaning)
Notebook for recording
Calculator (optional)
Procedure
Follow these steps carefully to estimate the cell size:
Step-by-Step Instructions
Clean the microscope: Use lens paper to clean the eyepiece and objective lenses.
Place the ruler on stage: Put the transparent ruler directly on the microscope stage. Make sure the
millimetre markings are clearly visible.
Focus the microscope: Look through the eyepiece and adjust the focus knobs until the ruler markings
appear sharp and clear.
Measure the field diameter: Observe how many millimetre divisions fit across the circular field of view.
Count carefully and record this measurement.
Convert to micrometres: Since cells are measured in micrometres (µm), convert your measurement using:
1 mm = 1000 µm
Remove ruler, place onion slide: Remove the ruler and place the prepared onion peel slide on the stage.
Focus until cells are clearly visible.
Count cells along diameter: Count how many cells fit in a straight line across the diameter of the
circular field. Count only complete cells.
Calculate cell size: Use the formula given below to estimate the size of one cell.
Formula for Cell Size Calculation
Estimation Formula
Estimated Cell Size = Field Diameter (µm) ÷ Number of Cells Along Diameter
Different students may get slightly different measurements. This is normal because:
Onion cells naturally vary in size (typically 150-300 µm)
Different microscopes may have different field diameters
Counting accuracy may differ
The important skill is learning how to estimate, not getting an exact number
Result
The estimated size of one onion peel cell is approximately 200 µm (or whatever your calculation shows).
This may vary between 150-300 µm depending on which part of the onion was used.
Inference
Based on this activity, we can conclude:
Onion peel cells are extremely small (around 200 µm = 0.2 mm)
Cells are far below the resolution limit of the human eye (100 µm or 0.1 mm)
A compound microscope is essential to see and study cells
The field of view method is a practical way to estimate cell size without complex instruments
Why This Method Works — Scientific Explanation
Understanding Resolution and Magnification
The resolution limit of the human eye is approximately 0.1 mm (100 µm). This means we cannot
see objects smaller than 100 µm clearly with our naked eye.
Why we need a microscope:
Typical onion cells = 150-300 µm (larger than resolution limit, but still too small to see details)
Most human cells = 10-30 µm (well below resolution limit)
Bacteria = 1-10 µm (impossible to see without microscope)
Historical Note
Robert Hooke first observed cells in 1665 using a simple microscope. He looked at thin slices of cork
and saw box-like structures, which he called "cells" because they reminded him of small rooms (cells) in a monastery.
Learn more about this discovery in Cell Theory.
Magnification Calculation
If you want to know the total magnification of your microscope:
Total Magnification = Eyepiece Magnification × Objective Magnification
For example: If eyepiece = 10× and objective = 40×, then total magnification = 10 × 40 = 400×
Precautions and Pro Tips
Precautions:
Handle the microscope with both hands carefully
Clean lenses before starting (fingerprints affect clarity)
Count cells in a straight line only (don't count diagonally)
Don't count partial cells at the edges of the field
Never touch lenses with fingers — use lens paper only
Pro Tips:
Use low magnification first, then switch to higher power
If cells overlap, choose a clearer area of the slide
Repeat the counting 2-3 times and take average
Work in good lighting conditions
Keep one eye closed while viewing through eyepiece
5 Important Viva Questions
Q1. What is the limit of resolution of the human eye? Why can't we see cells with the naked eye?
The limit of resolution of the human eye is approximately 0.1 mm or 100 micrometres (µm).
Why we can't see cells:
Most cells are smaller than 100 µm (the resolution limit)
Human cells typically range from 10-30 µm
Bacteria are even smaller (1-10 µm)
Even though onion cells (150-300 µm) are slightly larger than the resolution limit, their internal structures and
details are too small to see without magnification
A microscope magnifies cells hundreds of times, making them clearly visible
Q2. What unit is used to measure cell size? Why is this unit necessary?
Cell size is measured in micrometres (µm), also called microns.
Conversion:
1 micrometre (µm) = 0.001 millimetre (mm)
1 millimetre (mm) = 1000 micrometres (µm)
1 metre (m) = 1,000,000 micrometres
Why this unit is necessary:
Cells are extremely small — using millimetres would give decimal values (0.2 mm) which are inconvenient
Using micrometres gives whole numbers (200 µm) which are easier to work with
It's the standard international unit for microscopic measurements
Example: An onion cell is 200 µm, which is easier to say than 0.0002 metres or 0.2 mm
Q3. How do you calculate the total magnification of a compound microscope?
The total magnification of a compound microscope is calculated by multiplying the magnification of the eyepiece lens
by the magnification of the objective lens.
Total Magnification = Eyepiece Magnification × Objective Magnification
Examples:
Eyepiece 10× and Objective 10× → Total = 10 × 10 = 100×
Eyepiece 10× and Objective 40× → Total = 10 × 40 = 400×
Eyepiece 15× and Objective 100× → Total = 15 × 100 = 1500×
Note: Most school microscopes have 10× eyepiece and three objectives (10×, 40×, 100×), giving
total magnifications of 100×, 400×, and 1000× respectively.
Q4. Who first observed cells under a microscope? What did he observe?
Robert Hooke first observed cells under a microscope in 1665.
What he observed:
He examined thin slices of cork (dead plant tissue) under a simple microscope
He saw tiny box-like compartments arranged in rows
These compartments reminded him of small rooms or cells in a monastery
He named these structures "cells" — and this name is still used today
Important note:
What Hooke actually saw were dead cell walls of cork cells
The living contents of the cells had dried out, leaving only the rigid
cell walls
Later scientists discovered that living cells contain cytoplasm, nucleus, and
organelles
Hooke's discovery was the foundation of modern
Cell Theory
Q5. If the field diameter is 4 mm and 20 cells fit along the diameter, what is the estimated cell size?
Given:
Field diameter = 4 mm
Number of cells along diameter = 20 cells
Solution:
Step 1: Convert field diameter to micrometres
Field diameter = 4 mm
Since 1 mm = 1000 µm
Field diameter = 4 × 1000 = 4000 µm
Step 2: Calculate cell size using formula
Estimated Cell Size = Field Diameter ÷ Number of Cells
Estimated Cell Size = 4000 µm ÷ 20
Estimated Cell Size = 200 µm
Answer: The estimated size of each cell is 200 micrometres (µm) or 0.2 mm.
Key Terms to Remember
Micrometre (µm)
Unit of measurement for cells; 1 micrometre = 0.001 millimetre; also called micron.
Field of View
The circular area visible when looking through the microscope eyepiece.
Resolution
The smallest distance between two points that can be distinguished as separate; human eye resolution is ~100 µm.
Magnification
The process of enlarging the appearance of an object; calculated by multiplying eyepiece and objective lens powers.
Compound Microscope
A microscope with two or more lenses (eyepiece and objective) that provides high magnification.
Objective Lens
The lens closest to the specimen on a microscope; usually has magnifications of 10×, 40×, or 100×.
Eyepiece Lens
The lens you look through on a microscope; typically has 10× or 15× magnification.
Cell Size Estimation
Method of calculating approximate cell dimensions using field diameter and cell count.
