B. Life Systems · Cells

Using a Microscope: Parts, Magnification and Scientific Drawings

B1.1 A1.4 A1.5About 55 minutes

Learning goals

I can name the parts of a compound light microscope and explain what each one does.
I can calculate total magnification.
I can follow safe steps to focus a microscope and make a wet mount.
I can make a clear, labelled scientific drawing and estimate the size of a cell.

Words to know

compound light microscope
A microscope that uses light and two sets of lenses (eyepiece and objective) to magnify a specimen.

magnification
How many times larger an image looks than the real object. Written with ×, like 40×.

field of view
The circle of light you see when you look through a microscope.

specimen
The thing you are looking at, such as a piece of onion skin.

wet mount
A slide made by placing a specimen in a drop of water (or stain) and covering it with a cover slip.

resolution
How clearly a microscope shows two close-together points as separate. Higher resolution means sharper detail.

A whole world in a drop of water

Put a drop of pond water under a microscope and it comes alive: tiny creatures zoom, spin and glide. Scientists, medical lab technologists, veterinarians and water-quality inspectors use microscopes every day. In this lesson you will learn how a school microscope works, so that you can use one safely and well, in Grade 8 and in high school.

A school compound light microscope on a lab bench, with a dropper bottle and a box of glass slides beside it. — Find the parts you just learned: the eyepiece at the top, the objective lenses on the rotating nosepiece, the stage with its clips, the focus knobs and the light in the base. AI-generated image

A compound light microscope. Light from the light source passes up through the diaphragm, the slide and the specimen, then through an objective lens and the eyepiece to your eye.

Part	What it does
Eyepiece (ocular lens)	The lens you look through. It usually magnifies 10×.
Body tube	Holds the eyepiece at the right distance above the objective lenses.
Revolving nosepiece	Turns to switch between objective lenses.
Objective lenses	The lenses closest to the slide. Most school microscopes have 4× (low), 10× (medium) and 40× (high). The longer the lens, the higher its power.
Stage and stage clips	The flat platform that holds the slide; clips keep it still.
Diaphragm	Controls how much light passes up through the specimen.
Light source	A lamp (or mirror) that sends light up through the specimen.
Coarse focus knob	Moves the stage (or tube) a lot. Use it only with the low-power (4×) lens.
Fine focus knob	Moves things a tiny bit to sharpen the image. Use it with the 10× and 40× lenses.
Arm and base	The frame. Carry the microscope by the arm, with your other hand under the base.

Magnification

Each lens magnifies. To find the total magnification, multiply:

total magnification = eyepiece power × objective power

With a 10× eyepiece: low power is 10 × 4 = 40×, medium is 10 × 10 = 100×, and high is 10 × 40 = 400×.

What you see changes as you zoom in

The image is upside down and backwards. A letter e on the slide looks like an upside-down, backwards e. If you move the slide left, the image moves right.
At higher power, the field of view gets smaller. You see more detail, but less of the specimen, and the image is usually dimmer (open the diaphragm a little).
Always find the specimen on low power first, centre it, then switch to a higher power.

Worked example

Let's calculate magnification and field of view

A microscope has a 10× eyepiece. On low power (4× objective), the field of view is 4.5 mm across. (a) What is the total magnification on low power and on high power (40×)? (b) About how wide is the field of view on high power?

(a) Low power: 10 × 4 = 40×. High power: 10 × 40 = 400×.
(b) Going from 40× to 400× magnifies 10 times more (400 ÷ 40 = 10).
When magnification is 10 times more, the field of view is 10 times narrower: 4.5 mm ÷ 10 = 0.45 mm.
Change to micrometres (1 mm = 1000 µm): 0.45 mm = 450 µm.

(a) 40× and 400×. (b) About 0.45 mm (450 µm) across.

Worked example

Let's estimate the size of a cell

On high power the field of view is 450 µm across. You count about 5 onion cells lined up end to end across the field of view. About how long is one onion cell?

The cells fill the whole width: 5 cells take up 450 µm.
Divide: 450 µm ÷ 5 = 90 µm.
Check that it makes sense: 90 µm is 0.09 mm, too small to see clearly with your eyes alone, which fits.

Each onion cell is about 90 µm (0.09 mm) long. This is an estimate; real cells vary.

Zooming from 40× to 400× magnifies 10 times more, so you see a circle 10 times narrower: the small square on the left fills the whole view on the right. On high power, 5 onion cells across 450 µm means each cell is about 90 µm.

Making a wet mount

Place a clean slide on the table. Add one drop of water to the centre.
Use tweezers to place a thin specimen (for example, the see-through skin from inside an onion layer) flat in the drop.
Hold a cover slip at an angle, touching the drop, and lower it slowly so you don't trap air bubbles (they look like dark-edged circles).
Add a drop of stain (such as iodine) at one edge of the cover slip if your teacher provides it. Stains make the nucleus and cell walls easier to see.

Scientific drawings

Use a sharp pencil, no colouring or shading (use dots for darker areas).
Draw only what you see, large and clear, inside a circle for the field of view.
Label parts with straight, horizontal lines that don't cross. Labels go to the side.
Add a title, the specimen name and the magnification (for example: Onion epidermis cells, 400×).

Microscope safety and care

Carry it with two hands: one on the arm, one under the base.
Start on the lowest power. Never use the coarse focus knob on high power: the lens can smash into the slide.
When focusing, watch from the side as you bring the lens close to the slide, then look through the eyepiece and focus away from the slide.
Glass slides and cover slips can break. Tell an adult about broken glass; never pick it up with bare hands.
Never use a mirror microscope in direct sunlight: it can focus sunlight into your eye.
Stains like iodine can stain skin and clothes and are not safe to swallow. Wear gloves and goggles if your teacher asks you to.

Just for fun

What did the slide say to the 40× objective lens?

"Back off! Use the fine focus knob!" (Slides can't really talk, but if they could, that's what they'd say.)

Hands-on activity

Build a water-drop magnifier

You don't need a real microscope to explore magnification. You need: clear plastic wrap, a small clear plastic lid or a piece of stiff clear plastic (for example, from packaging), a drinking straw or an eyedropper, water, and a page of newspaper or a magazine with small print. A smartphone camera is optional.

Stretch plastic wrap tightly over the opening of a clear container lid, or use the stiff clear plastic.
Use the straw or dropper to place one small drop of water on top. The curved drop acts like a lens.
Hold it over the small print. Raise and lower it slowly until the letters are sharp. That's focusing!
Look at the letter e. Is it right side up? (A single drop usually shows it right side up and larger. In a compound microscope, the objective lens flips the image.)
Try a smaller drop and a bigger drop. Which magnifies more? (Usually the smaller, more curved drop.)
Look at a leaf, salt grains, a strand of hair and the screen of a phone. Make a scientific drawing of one of them, with title and labels.
Optional: hold a phone camera right behind the drop and take a photo.

What to look for: a careful drawing in pencil with straight label lines; the observation that a more curved (smaller) drop magnifies more. On a phone or TV screen you may see the tiny coloured dots or bars that make up the picture.

Your turn

An eyepiece is 10× and the objective is 10×. What is the total magnification?

Which focus knob should you use with the 40× objective?

Why should you always start on the lowest power objective?

You move the slide to the right. Which way does the image move?

What do air bubbles look like in a wet mount, and how can you avoid them?

Give three rules for a good scientific drawing.

What does the diaphragm control?

More practice: warm-up

Which part do you look through?

Which part holds the slide?

10× eyepiece and 4× objective: total magnification?

How many micrometres are in 1 mm?

How should you carry a microscope?

More practice: on your own

A microscope has a 15× eyepiece and a 40× objective. What is the total magnification?

On 40× the field of view is 5 mm. What is it on 100×?

The field of view is 2 mm (2000 µm). About 40 cheek cells fit across it. Estimate the size of one cell.

Switching from 10× to 40× objective, the image becomes darker. Why, and what can you do?

A student's drawing is coloured in, has crossing label lines and no magnification. Give three ways to improve it.

Challenge

On high power (400×) the field of view is 0.4 mm. A paramecium stretches about half of the way across. About how long is it, in µm?

A photo of a cell in a textbook is 40 mm wide. The caption says magnification 400×. How wide is the real cell, in µm?

Why does the field of view shrink when magnification goes up? Use an everyday example to explain.

Check your understanding

Quiz

A microscope has a 10× eyepiece and a 40× objective. The total magnification is…

Which part should you never use on high power?

As you change to a higher-power objective, the field of view…

On low power the field of view is 4 mm. Eight cells fit across it. Each cell is about…

Which is a rule for a scientific drawing?

The image in a compound microscope is…

For parents and teachers

Big idea: a microscope is a technology that made cell biology possible. Using it well means knowing its parts, calculating magnification and following safety rules.

How to help at home:

Build the water-drop magnifier together and compare different drop sizes.
Practise the math: total magnification (multiply), and changing between mm and µm (× or ÷ 1000).
If you own a toy or student microscope, set it up away from direct sunlight and supervise any stains.

Safety: glass slides and cover slips break easily; adults handle broken glass. Mirror-type microscopes must never point at the sun. Stains such as iodine are not to be swallowed and can stain skin and clothing.

Common difficulties:

Adding instead of multiplying the lens powers.
Forgetting that the field of view gets narrower as magnification goes up.
Mixing up mm and µm. Remind them: 1 mm = 1000 µm.