KS3 · Biology

Preparing and observing a microscope slide

A lump of onion under a microscope shows you nothing — light can't get through it. Make it thin, stain it, lower the glass slowly, and suddenly: cells.

Making and viewing an onion slide

Seven steps, in order. Tap a step marked + to see why it matters — that's the part that makes the method make sense.

  2. Lay it flatPlace the sample flat on a rectangular glass microscope slide.
  5. Soak up the extra stainUse a little tissue to absorb any excess stain.
  6. Focus on low powerStart on the lowest magnification. Adjust the focus until you can see the tissue, then use the fine focus to make it clear.
  7. Zoom inChange to a higher objective lens to increase the magnification. Now the stained onion cells show their cell walls and a nucleus.

Onion slide vs cheek slide

Onion (plant cells)vsCheek (animal cells)

Plant cells or animal cells: the method has the same backbone, but three things change.

Focus

The stain

Onion (plant cells)

Iodine

Cheek (animal cells)

Methylene blue

The insight

This is the swap people mix up most. Iodine goes on onion cells; methylene blue goes on cheek cells.

Getting the cells

Onion (plant cells)

Tweezers take a thin layer of tissue from between the onion's layers and lay it flat on the slide.

Cheek (animal cells)

Rub a cotton bud over the inside of your cheek, then rub it over the middle of the slide to transfer the cells.

Extra hygiene step

Onion (plant cells)

Not part of the onion method.

Cheek (animal cells)

Put the used cotton bud into disinfectant before you add the stain.

Coverslip and tidy-up

Onion (plant cells)

Tweezers, then lower it slowly with a mounted needle. A little tissue absorbs the excess stain.

Cheek (animal cells)

Exactly the same.

Spot the slip

Find the step that spoiled Sam's slide

Sam made an onion slide, but big black circles got in the way of the cells. Which line of the method went wrong?

Sam's write-up — which line goes wrong?

Recording what you see

Scientific drawing: follows the rules, or breaks them?

Put each feature of a drawing of onion cells where it belongs.

Still to sort

Follows the rules (0)

Keeps the drawing clear and simple.

Breaks the rules (0)

Stops the drawing being clear and simple.

10 of 10 still to sort.

A scientific drawing is a clear and simple line drawing of what you saw — not a work of art.

Predict, then check

Commit to an answer before you look.

You're looking at onion cells through a ×10 eyepiece lens. You switch to the ×40 objective lens to zoom in. What is the total magnification now?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Thin enough for light, stained so the cells show, no bubbles in the way.

What you need to know

  • A tissue is a group of similar cells with the same job, working together. A thin sample of tissue can be put on a slide and viewed with a light microscope.
  • Making a slide: thin layer of specimen on a glass slide → stain → coverslip lowered slowly with a mounted needle → excess stain soaked up with tissue.
  • The reasons: thin so light can pass through; stain so the cells and their structures show; lowered slowly so air bubbles move out.
  • Iodine stains onion (plant) cells. Methylene blue stains cheek (animal) cells, and the cotton bud goes into disinfectant.
  • Observe on the lowest magnification first, focus, use the fine focus, then switch to a higher objective lens. Record it as a scientific drawing and write the magnification.

The big picture

A microscope slide holds a thin layer of specimen on glass, with a stain added and a thin glass coverslip on top. Every step has a reason: the sample is as thin as possible so light can pass through it, the stain makes the cells and their structures easier to see, and the coverslip is lowered slowly so air bubbles escape. Onion (plant) cells are stained with iodine; cheek (animal) cells with methylene blue, and the cheek cotton bud goes into disinfectant. You observe on the lowest magnification first, focus, then zoom in; record what you see as a scientific drawing; and work out magnification = eyepiece × objective.

Key points

1The specimen must be as thin as possible — ideally one cell thick — so that light can pass through it.
2A stain is a coloured liquid that makes cells and their structures easier to see. Stains can stain skin and clothes, so handle them with care.
3Lower the coverslip slowly with a mounted needle so air bubbles move outwards. Trapped bubbles look like big black circles and block the view.
4Onion cells: tweezers and iodine. Cheek cells: cotton bud, disinfectant for the used bud (bacteria and viruses can transfer from our mouths), and methylene blue.
5Start on the lowest magnification, focus until you can see the tissue, sharpen it with the fine focus, then change to a higher objective lens. Stained onion cells show cell walls and a nucleus.
6A scientific drawing: two or three cells, drawn at least half a page big, smooth joined-up lines, stippling not shading, labels, and the magnification.
7Magnification = eyepiece lens magnification × objective lens magnification. The eyepiece is usually ×10.

Worked example

Problem

Ana has drawn onion cells that she saw through a ×10 eyepiece lens and a ×10 objective lens. What magnification should Ana write next to her drawing?

⚠ Watch out

Adding the two lens magnifications instead of multiplying them. Magnification = eyepiece lens × objective lens, every time — the lenses never add.

🧠

Memory hook

THIN, TINT, SLOW: thin so the light gets through, tint (stain) so the cells show up, slow with the coverslip so the bubbles get out.

✓

Check yourself

Cover the page. Give the reason for three steps: why the onion sample is thin, why you add a stain, and why the coverslip goes down slowly.

Flashcards

(15)
What is a tissue?
A group of similar cells with the same job, working together.
Why must the specimen on a slide be as thin as possible?
So that light can pass through it. Ideally it is just one cell thick.
What is a stain, and why is it used?
A coloured liquid put onto a specimen so the cells and their structures are easier to see.
Which stain is used for onion cells, and which for cheek cells?
Iodine for onion (plant) cells; methylene blue for cheek (animal) cells.
Why are stains handled with care?
They can stain your skin and clothes.
Which tool lowers the coverslip onto the slide?
A mounted needle — used to lower the coverslip gently.
What do air bubbles trapped under a coverslip look like?
Big black circles that get in the way of the view of the cells.
How are cheek cells collected for a slide?
Rub a cotton bud over the inside of your cheek, then rub it over the middle of the slide.
Why does the cotton bud go into disinfectant?
Bacteria and viruses can transfer from our mouths.
Which magnification do you start on?
The lowest. Focus until you see the tissue, use the fine focus, then change to a higher objective lens.
What can you see in stained onion cells?
Cell walls and a nucleus.
How many cells go in a scientific drawing, and how big?
Two or three cells, drawn at least half a page big.
What kind of lines and shading does a scientific drawing use?
Smooth, continuous, joined-up lines — and stippling (tiny dots), never shading.
Apart from the cells, what must a scientific drawing include?
Labels on the structures you know, and the magnification.
How do you calculate the total magnification?
Eyepiece lens magnification × objective lens magnification.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.