S1 Science | Microscope Work and Cell Observation: a Tutor Prep Guide | Magnification, field of view, drawing rules

2026-11-09

Tutor lesson-prep guides · Secondary Biology

The student grabs the coarse knob, crushes the slide, then says nothing can be seen. The problem is not curiosity; nobody has told them the order: low power first, coarse before fine, light before focus.

Objectives — By the end the student can: (1) multiply eyepiece by objective to get total magnification and explain why a higher power shows a smaller field; (2) convert a 50 micrometre specimen at 400 times into a 20 millimetre image, and convert back; (3) draw one observed cell using the three drawing rules (pencil, clear single lines, labels on straight non-crossing lines) and state the magnification.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit conversion	How many micrometres are in one millimetre	one thousand, and the direction is stated
Ratio	An image at 400 times is how many times the real length	400 times, by multiplication not addition
Care of equipment	How do you carry a microscope to another bench	one hand on the arm, one under the base, tube upright

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Look first	Hand out an onion epidermis slide and ask for a naked-eye description	Write down three visible features
8-20 min	Parts and order	Name the parts from base to eyepiece, show low power then coarse then fine	Label five parts and put the steps in order
20-34 min	Magnification	Write eyepiece 10 and objective 40, ask for the total	Work out 400 and explain the multiplication
34-46 min	Field of view	Same slide from 100 to 400 times, field was 1.8 mm, now what	Get 0.45 mm and explain why it shrank
46-56 min	Drawing rules	Draw one cell, stating pencil, line and label rules	Copy the drawing and check each other
56-60 min	Exit check	Collect three questions and mark at once	Work independently

03 Teaching script (read it out)

Scene 1: Eyepiece times objective

Total magnification is the eyepiece power multiplied by the objective power, never added. A 10 times eyepiece with a 40 times objective gives 400 times.
Write 10 times 40 on the board, then ask how to get 100 times. Students will offer a 10 times objective, and at that moment they see the two lenses are independent.
The sentence they must own: changing either lens changes the total magnification.

Ask the student to repeat: total magnification is eyepiece power times objective power.

Scene 2: Higher power, smaller field

The higher the power, the smaller the area you see. At 100 times the field is 1.8 mm across; at 400 times it is only 0.45 mm, which is 450 micrometres.
The usual misconception is that more power means seeing more. It is the opposite: more power only enlarges a tiny patch.
Use two paper circles, one inside the other, so the trade-off is visible; that is why we search at low power first.

Ask the student to repeat: higher power shows a smaller field, so find the target at low power first.

Scene 3: Real size and image size

A magnification of 400 means the image is 400 times the real length. A 50 micrometre cell is drawn 50 times 400, which is 20000 micrometres, or 20 millimetres.
The reverse: measure a 2.0 mm image, divide by 400 to get 0.005 mm, which is 5 micrometres.
Always write the unit. A micrometre is one thousandth of a millimetre; skip that step and the answer is out by a thousand.

Ask the student to repeat: image divided by magnification is real size; real size times magnification is image size.

04 Question types and worked steps

Type	Example	Steps	Answer
Total magnification	Eyepiece 10, objective 40, total power	Multiply the two powers, then note there is no unit	400 times
Real size to image	A 50 micrometre cell at 400 times, image length	50 times 400 in micrometres, then divide by 1000 for millimetres	20000 micrometres, or 20 mm
Image to real size	Image measures 2.0 mm at 400 times, real size	Convert mm to micrometres first, then divide by the power	5 micrometres
Field of view	Field is 1.8 mm at 100 times; at 400 times it is	Field varies inversely with power: 1.8 times 100 divided by 400	0.45 mm, or 450 micrometres

Sanity check (teach this): Sanity checks: the total is always larger than either power because the lenses multiply; an image is longer than the specimen, so finding real size always means dividing and the answer must be smaller; the field of view must shrink at higher power, so a larger answer means the ratio is upside down; micrometres and millimetres differ by a factor of a thousand, so check units whenever a number looks wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
Eyepiece 10 with objective 10 gives a field of 1.8 mm; what is it at objective 40	0.45 mm	Whether 1.8 was multiplied by 40 and divided by 10, and whether the unit is stated
A cell is 60 micrometres across; drawn at 400 times, how many millimetres	24 mm	Whether the divide by 1000 was forgotten, or 60 times 4 used instead of 400
An image is 3.0 mm long at 300 times; real length in micrometres	10 micrometres	Whether 3.0 mm was converted to 3000 micrometres before dividing by 300

06 Common mistakes and what to say

Mistake	Student writes	Say this
Adding the two powers	The total is 10 plus 40, so 50 times	The lenses multiply, not add: 10 times 40 is 400. Using 50 makes every size eight times too small.
Thinking a higher power sees more	With the 40 objective the field got bigger	It is the other way round: higher power, smaller field. Find the target at low power, then switch up.
Skipping the unit conversion	50 times 400 is 20000 millimetres	20000 is micrometres. One millimetre is 1000 micrometres, so write 20 mm.
Using the coarse knob at high power	Twisting the coarse knob straight after switching to 40	At high power use the fine knob only, and lower the tube while watching from the side, not through the eyepiece.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — All chapters in the subject
Classification and dichotomous keys — The same observation and recording skills
Redo this chapter — Recalculate field and image size for three lens pairs

08 Exit check (last 5 minutes)

Question	Answer	Decision
Total power with eyepiece 10 and objective 10	100 times	Answering 20 means the powers were added; retake the first act
Image length in mm for a 50 micrometre cell at 400 times	20 mm	Answering 20000 means the unit was not converted; retake the third act
Field of view in mm at 400 times when it is 1.8 mm at 100 times	0.45 mm	Answering 7.2 means the ratio is upside down; retake the second act

09 Tutor FAQ

Do we need 400 times to see cells?

No. Onion epidermis cells are already visible as packed rectangles at 100 times; 400 times shows the nucleus and the cell wall thickness more clearly. Search low, then switch up.

Must S1 students name every part?

They should recognise the eyepiece, objective, stage, coarse knob and fine knob, and state the focusing order; the rest can be matched to the real instrument.

May I draw with a ballpoint pen?

No. Biological drawings use pencil, with clear single lines, straight non-crossing label lines, and the magnification written underneath.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.