What is a microscope and what are its parts?
A microscope is a tool that makes very small things look big. Microscopy means using a microscope to study things.
A school compound light microscope has these main parts:
- Eyepiece (ocular lens): the lens you look into. Usually 10×.
- Objective lenses: the lenses close to the sample. Usually 4×, 10×, 40× and 100×. They sit on a turning disc called the nosepiece.
- Stage: the flat platform that holds the slide, with clips.
- Light source or mirror: sends light up through the sample.
- Diaphragm: controls how much light passes.
- Coarse and fine focus knobs: move the stage or tube to make the picture sharp.
- Arm and base: hold everything; carry the microscope by these.
It is called compound because it uses two lenses one after the other. A single magnifying glass is a simple microscope.
Types of microscopes
Light (optical) microscopes
They use visible light and glass lenses. They can show living cells, and colours from stains. Best detail: about 200 nm (0.2 µm). Types include the simple microscope, the compound microscope, the stereo (dissecting) microscope for whole insects or leaves, and the fluorescence microscope that makes tagged parts glow.
Electron microscopes
They use a beam of electrons and magnets instead of light and glass. Electrons have a far shorter wavelength, so the detail is much finer: down to about 0.1 nm.
- TEM (transmission electron microscope): electrons pass through a very thin slice. Shows the inside of cells: mitochondria, ribosomes.
- SEM (scanning electron microscope): electrons bounce off the surface. Gives 3D-looking pictures of pollen, insect eyes.
Limits: samples must be dead and placed in a vacuum, the machine is very costly, and pictures are black and white (colours are added later on a computer).
Magnification and resolution
Magnification tells how many times bigger the image looks than the real thing.
Total magnification = eyepiece power × objective power
Example: 10× eyepiece with a 40× objective gives 400×.
You can also use: magnification = image size ÷ actual size. Keep both in the same unit. 1 mm = 1000 µm, and 1 µm = 1000 nm.
Resolution (resolving power) is the smallest gap between two points that still shows them as two separate points. Better resolution means a smaller number and a clearer picture.
Making a picture bigger without better resolution only makes it a bigger blur. This is called empty magnification. That is why light microscopes stop being useful after about 1000–1500×.
Slide preparation and staining
A temporary wet mount is the most common slide in school labs.
- Clean a glass slide.
- Put one drop of water (or glycerine) in the centre.
- Place a very thin sample in the drop, for example one layer of onion peel. Light must pass through it.
- Add a drop of stain: iodine or safranin for plant cells, methylene blue for cheek cells. Stain colours parts like the nucleus so you can see them.
- Lower a cover slip at an angle using a needle. This pushes air out so no bubbles form.
- Soak extra liquid with blotting paper.
- Start with the lowest objective (4×), focus with the coarse knob, then switch to higher power and use only the fine knob.
A permanent slide is dried, stained, and sealed so it lasts for years.
Try it: a water-drop magnifier
Put a clear plastic sheet over a newspaper. Place one small drop of water on it. Look at the letters through the drop. They look bigger! The curved drop acts like a tiny lens. A smaller, rounder drop magnifies more. This is how the very first simple microscopes worked, using tiny glass beads.
Key formulas and definitions
- Total magnification = eyepiece power × objective power
- Magnification = image size ÷ actual size
- Actual size = image size ÷ magnification
- 1 mm = 1000 µm; 1 µm = 1000 nm
- Resolution limit: light ≈ 200 nm; electron ≈ 0.1 nm
Worked examples
1. The eyepiece is 10× and the objective is 100×. Find the total magnification.
Total = 10 × 100 = 1000×.
2. A microscope gives 400× with a 10× eyepiece. Which objective is in use?
Objective = 400 ÷ 10 = 40×.
3. A cell appears 8 mm wide in a photo taken at 400×. What is its real width in µm?
Real size = 8 mm ÷ 400 = 0.02 mm. Change to µm: 0.02 × 1000 = 20 µm.
4. A bacterium is 2 µm long. Its image is 10 mm long. Find the magnification.
Same units: 10 mm = 10,000 µm. Magnification = 10,000 ÷ 2 = 5000×.
5. Two organelles are 50 nm apart. Can a light microscope show them as two?
No. A light microscope can only separate points about 200 nm apart. 50 nm is smaller, so they look like one. An electron microscope (≈ 0.1 nm) is needed.
6. Why do bubbles appear on some slides, and how do you avoid them?
Air gets trapped when the cover slip is dropped flat. Touch one edge of the cover slip to the drop and lower it slowly at an angle with a needle, so the liquid pushes the air out.
Common mistakes
- Adding the powers (10 + 40 = 50) instead of multiplying (10 × 40 = 400).
- Thinking more magnification always means more detail. Without better resolution it is just a bigger blur.
- Mixing units in image size ÷ actual size. Change mm to µm first.
- Using the coarse knob at high power. It can crack the slide; use only the fine knob at 40× and 100×.