How to Study Cells — Microscopes & Magnification
Why Can't We See Cells with the Naked Eye?
Most plant and animal cells are 10–100 µm in size. The human eye can distinguish two points only if they are at least 0.1 mm (100 µm) apart — this is called the limit of resolution of the human eye.
The minimum distance between two points at which they appear as separate and distinct. For the human eye: 0.1 mm at 25 cm distance.
Robert Hooke was the first person to observe cells in 1665 using his self-designed microscope (200–300X magnification). He observed cork and named the box-like compartments "cells".
Types of Microscopes
| Feature | Light Microscope | Electron Microscope |
|---|---|---|
| Uses | Visible light | Beam of electrons |
| Resolution | ~200 nm (0.2 µm) | ~0.1 nm (sub-nanometre) |
| Can see | Most cells, nucleus | Ribosomes, viruses, membranes |
| School use | Yes (10X, 40X objective) | No (research only) |
Total magnification = eyepiece magnification × objective lens magnification. E.g., 10X eyepiece × 10X objective = 100X total.
Fig. 2.1: Structure of a light microscope — Parts: Eyepiece, Body tube, Objective lens, Stage, Mirror, Coarse & Fine adjustment knobs
Fig. 2.2: Size scale of objects — Atoms (0.1 nm) to human height (1.8 m). Most cells visible only under light microscope (10–100 µm)
Worked Example: Estimating Cell Size Under Microscope
🧠 Memory Trick — Hooke & Microscopes
Hooke observed Cork in 1665 → named Compartments as Cells → Resolution limit = 0.1 mm for naked eye → 0.2 µm for light microscope → 0.1 nm for electron microscope
Magnification formula: Total Mag = Eyepiece × Objective. Remember: Light microscope uses light; Electron microscope uses electrons!
📌 NEET/JEE Tip
NEET directly asks: "Who discovered cells?" (Robert Hooke, 1665, cork). "Resolution of human eye?" (0.1 mm). "Which microscope reveals ribosome structure?" (Electron microscope). Magnification calculation is a guaranteed 1-mark MCQ every year. Remember: 1 mm = 1000 µm = 1,000,000 nm.
Practice Questions — Microscopes & Cell Study
Q1. Robert Hooke observed cells in 1665 by examining thin slices of:
Q2. A microscope has a 10X eyepiece and a 40X objective. The diameter of the field is 2 mm. If 20 cells are seen along the diameter, what is the estimated size of one cell?
Q3 (HOTS). An electron microscope is used to study ribosomes. Why can't a light microscope be used for this purpose?
