Cells and organisation · Investigation
Using a microscope
Turn it up to the highest magnification and you see almost nothing. Why?
Start here
Same slide. Same microscope. Same person.
Two photographs, taken one minute apart, of one piece of onion skin. In the first you can count the cells. In the second there is a grey smear with one dark curve in it. Nothing was changed except the objective lens.
Which objective would you turn in first to find the cells?
Hold that. You will measure it yourself further down, and the number that settles it is how much of the slide you can see at once.
Your turn · judge it first
Sam’s method
Sam wrote this up for a slide of onion skin. Three of the six steps will cost him. Tap the ones you would change, then open them up.
Costs him Dropped flat, it traps air. Bubbles are round, thick and black-edged, and they get drawn as cells every year. Lower it slowly on one edge instead.
Sound Light has to come up through the specimen, so it must sit over the hole.
Costs him That is 0.45 mm of slide in view. He is hunting for something the width of a hair in the dark. Start on ×4, where he can see 4.5 mm at once.
Costs him From the eyepiece he cannot see the gap closing. Watch from the side, bring it down until it nearly touches, then focus by moving it away.
Sound A biological drawing is a record, not a picture. Single lines, no shading, no colouring in.
Sound Without it the drawing says nothing about size, and no one can repeat what he did.
total magnification = eyepiece × objective
The triangle
Cover the one you want
Total is on its own at the top, with the other two side by side underneath. Cover it and you are left with eyepiece × objective — multiply.
Cover the eyepiece and you are left with total over objective. Divide the total by the objective.
Cover the objective and you are left with total over eyepiece. Divide the total by the eyepiece.
Two things side by side means multiply. One thing over another means divide.
Key fact
Multiply the two, never add them. ×10 and ×40 give ×400, not ×50.
Watch it done · one step at a time
Riya’s eyepiece says ×10. The objective clicked into place says ×40. Calculate the total magnification.
Formula
total magnification = eyepiece × objective
Written down before any numbers. That is what stops the next step going wrong.
Insert
total magnification = 10 × 40
Eyepiece where eyepiece goes, objective where objective goes.
Fine-tune
nothing to convert — both are already “times”
Multiply, never add. 10 + 40 = 50 is the commonest wrong answer on this calculation.
Answer
×400
Magnification has no unit — ×400 means four hundred times bigger, never 400 mm.
Now you · same four steps
Your eyepiece says ×10. You click the ×4 objective into place. Calculate the total magnification.
Write each step out. Then check your working against Riya’s and tick what you did.
Riya’s working, with your numbers
- F total magnification = eyepiece × objective
- I total magnification = 10 × 4
- F nothing to convert — multiply, do not add
- A ×40
Did you do all of these?
All 4 ticked — you can do this one on your own.
Did you say all of these?
- I wrote the formula out before I put any numbers in.
- I put 10 in for the eyepiece and 4 in for the objective.
- I multiplied. I did not add.
- My answer is written ×40 — a number of times, with no unit after it.
Investigate
The bench microscope
Turn from the ×4 objective to the ×40. What happens to the amount of slide you can see?
Make your prediction first — then the lab runs.
Think again
“The highest magnification always shows you the most.”
Magnification makes things bigger. It does not make them easier to find, and it takes away depth as fast as it adds size. Your own readout says it: at ×400 the field of view is 0.45 mm — fewer than two onion cells across — and the slice in focus is thinner than one cell.
So the method is: find it on the lowest power, get it in the middle, then climb.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
You have just clipped a new slide onto the stage. Which objective do you turn in first?
Rung 2 · The one that catches people
A microscope has a ×15 eyepiece. You click the ×40 objective into place. What is the total magnification?
Rung 3 · Explain
Ade finds his cells on ×4, then turns straight to ×40 and says they have vanished. Explain what has happened and what he should do.
Rung 4 · Take it somewhere new
Two students disagree about whether onion cells from the outer layer are longer than cells from an inner layer. Write the method you would give them.
Key note
Total magnification = eyepiece × objective. Every step up in magnification hands back field of view and depth, so find it low and climb.
Going further
Where does 180 come from? Printed on the eyepiece is a field number — on these ones, 18. Divide it by the objective and you get the field of view in millimetres. Because this eyepiece is ×10, dividing 180 by the total magnification gives the same answer. Change the eyepiece and the 180 changes with it.
Before this lesson
At GCSE this becomes
Where to next
Stuck on the calculation?
Microscope lamps get hot and slides are glass. Follow your teacher’s instructions at the bench.
Lesson content © MrBadmusAI.