2.1 How to Study Cells?

Last updated at July 10, 2026 by

๐Ÿ’ฌ Think about it

Hold your phone far away and the tiny letters blur into one blob. Bring it close and you can read again. Your eyes have a limit. Cells are far smaller than that limit. So how do scientists ever see them? Let us find out.

Image: A size scale chart from 1 kilometre down to 0.1 nanometre, showing where a rocket, a tree, a human, a chicken egg, plant and animal cells, bacteria, viruses and atoms fall, with bars marking the unaided eye, light microscope and electron microscope ranges.
Fig. 2.1 as a Scale โ€” What Each Tool Can See
Size Example object Seen with
1 km โ€“ 1 m Rocket, Neem tree, human height, Great Indian Bustard Unaided eye
0.1 m โ€“ 1 cm Chicken egg Unaided eye
1 mm Fish egg, Amoeba Unaided eye / light microscope
100 ยตm โ€“ 10 ยตm Most plant and animal cells, nucleus Light microscope
1 ยตm Most bacteria, mitochondrion Light microscope
100 nm โ€“ 10 nm Smallest bacteria, viruses, ribosomes Electron microscope
1 nm โ€“ 0.1 nm Proteins, lipids, small molecules, atoms Electron microscope
What is the limit of resolution of the human eye?
Limit of Resolution = 0.1 mm From about 25 cm, the eye can separate two points only if they are at least 0.1 mm apart; closer than that they look like one point. e.g. Two tiny dots drawn very close merge into one when seen from far.
Image: Two tiny dots drawn close together on paper, shown merging into a single dot when viewed from about 25 cm, illustrating the 0.1 mm limit of resolution of the human eye.
  • Resolution means seeing two close points as separate.
  • From about 25 cm, our eye separates points 0.1 mm apart.
  • Closer than that, they look like a single point.
  • So the limit of resolution of our eye is 0.1 mm.
Example: Two tiny dots drawn very close on paper merge into one dot when seen from far.
Why can't we see a cell with our naked eye?
Why a Cell is Invisible Cell is smaller than 0.1 mm Below the eye's limit A microscope is needed
Image: A comparison showing a human eye unable to see a tiny single cell on one side, and the same cell appearing large and clear when viewed through a microscope on the other side.
  • A cell is usually much smaller than 0.1 mm.
  • So it is below the limit of our eye.
  • We need tools that make small things look bigger.
How does a microscope help us see tiny things?
How a Microscope Helps Cells too small for the eye A lens magnifies the object Objective + eyepiece reveal cells
Image: A microscope with its eyepiece and objective lens labelled, showing how the two lenses together magnify a small object to make it appear much larger and clearer.
  • A convex lens, or a set of lenses, magnifies an object.
  • Magnification makes a small object appear larger.
  • A microscope uses an objective lens and an eyepiece together.
Example: A magnifying glass is a single convex lens that makes print look bigger.
Who first saw and named cells?
Robert Hooke (1665) He used a self-designed microscope and saw box-like compartments in a thin slice of cork. He named these boxes 'cells'.
Image: A historical illustration of Robert Hooke looking through his early microscope at a thin slice of cork, with a drawing of the box-like cork cells he observed and named cells in 1665.
  • Robert Hooke was the first to observe a cell in 1665.
  • He used a self-designed microscope (about 200โ€“300X).
  • He saw box-like compartments in a thin slice of cork.
  • He named these boxes "cells".
What are the main parts of a light microscope?
Parts of a Light Microscope Eyepiece The lens you look through at the top. Body Tube Connects the eyepiece to the objective lens. Objective Lens The lens near the object โ€” e.g. 10x, 40x. Coarse & Fine Knobs Move the lens up and down to focus the image. Stage Flat platform that holds the glass slide. Mirror / Light Source Sends light through the slide and lens.
๐Ÿ“ท Image placeholder โ€” upload a photo of a light microscope Image: A student looking through a light microscope in a school lab, with the microscope's eyepiece, body tube, objective lens, stage and mirror visible.
  • It has an eyepiece, a body tube and objective lenses.
  • Knobs (coarse and fine) focus the image.
  • A stage holds the slide; a mirror sends in light.
Part Job
Eyepiece The lens you look through at the top.
Body tube Connects the eyepiece to the objective lens.
Objective lens The lens near the object (e.g. 10X, 40X).
Coarse & fine knobs Move the lens to focus the image.
Stage The flat platform that holds the slide.
Mirror Reflects light up through the object.
๐Ÿ”ง Activity 2.1 โ€” Let us estimate the size of a cell

In this Activity, we will measure the field of view of a microscope and use it to estimate the real size of one onion peel cell.

Activity 2.1 โ€” Steps Step 1 Place a mm ruler on the stage; measure the field-of-view diameter. Step 2 Convert the diameter from mm to micrometre (x1000). Step 3 Replace the ruler with an onion peel slide. Step 4 Count the cells across the diameter in a straight line. Cell size = field diameter / number of cells
Image: A microscope with a transparent millimetre ruler on its stage, and a second view showing an onion peel slide with several box-like cells lined up across the circular field of view.
Materials needed
A microscope, a transparent ruler with mm markings, and an onion peel slide.
Procedure
1. Take a transparent ruler with millimetre (mm) markings.
2. Place the ruler on the stage, focus, and observe the diameter of the circular field of view through the eyepiece; measure it in mm.
3. Convert the diameter from mm to micrometre (ยตm). Suppose it is 5 mm, so 5 ร— 1000 = 5000 ยตm.
4. Remove the ruler and place an onion peel slide on the stage.
5. Focus on the slide and count the number of cells along the diameter in one straight line.
6. Estimate the real size of the cell using the formula.
Observation
Estimated size of onion peel cell = Diameter of the field in ยตm รท Number of cells along the diameter. If 25 cells are seen, size = 5000 ยตm รท 25 = 200 ยตm.
Explanation
A microscope makes a 200 ยตm cell appear larger. If both eyepiece and objective are 10X, the total magnification is 100X, so the cell looks 100 times bigger and clear.

โ†’ A microscope reveals very small structures clearly.

โ—† Summary
  • Measure field width.
  • Count cells across.
  • Divide to get size.
  • Microscope magnifies it.
What is an electron microscope?
Electron Microscope Electron uses an electron beam Microscope magnifies tiny objects A very powerful microscope that uses a beam of electrons to show fine cell structures at the nanometre scale.
  • It is a very powerful microscope.
  • It uses a beam of electrons, not light.
  • It shows fine cell structures at the nanometre scale.
  • A nanometre is one-billionth of a metre.
Image: A large laboratory electron microscope, a tall instrument with a column and a screen, used by scientists to view very fine details of cell structures using a beam of electrons.
Light microscope
Electron microscope
Uses light (visible).
Uses a beam of electrons.
Used in school labs.
Used by scientists for fine detail.
Shows cells and large parts.
Shows tiny parts at nanometre scale.
Important Points
  • The limit of resolution of the human eye is 0.1 mm.
  • Magnification = magnifying power of eyepiece ร— objective lens.
  • Scientists improved microscopes by improving resolution, contrast and magnification.
๐Ÿ’ก Ready to Go Beyond
Image: A scanning electron microscope image of the lower surface of a Colocasia leaf showing many tiny mouth-like openings called stomata scattered across the leaf surface.
  • Electron microscopes use a beam of electrons, not light.
  • They reveal very fine details of a cell.
  • They can show structures at the nanometre scale.
  • A nanometre is one-billionth of a metre.
โ“ Test Yourself
  1. What is the limit of resolution of the human eye?
    View Answer Hide Answer
    About 0.1 mm. Two points closer than this look like one.
  2. Who first observed and named cells?
    View Answer Hide Answer
    Robert Hooke, in 1665, while looking at a slice of cork.
  3. What two lenses give a microscope its magnification?
    View Answer Hide Answer
    The objective lens and the eyepiece.
  4. If both lenses are 10X, what is the total magnification?
    View Answer Hide Answer
    100X. The cell appears 100 times larger.
  5. Which microscope uses electrons instead of light?
    View Answer Hide Answer
    The electron microscope. It shows the finest details.
Important Definitions
  • Limit of resolution โ€” the smallest distance at which two points are seen as separate (0.1 mm for the eye).
  • Magnification โ€” making a small object appear larger using lenses.
  • Light microscope โ€” a tool that uses light and lenses to view small objects.
  • Electron microscope โ€” a tool that uses a beam of electrons to see very fine details.
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