Q1.
Under the microscope, you will see a magnified image of an object. Can you estimate its actual size?
Answer
Yes. Use the microscope's own field of view as a ruler.
- Place a transparent millimetre ruler on the stage and measure the diameter of the circular field of view — say 5 mm.
- Convert it: 5 mm × 1000 = 5000 µm.
- Replace the ruler with the specimen slide and count how many cells fit across that same diameter in one straight line — say 25.
- Divide.
estimated size of one cell = diameter of the visible field in µm ÷ number of cells along the diameter
= 5000 µm ÷ 25
= 200 µm (that is 0.2 mm)
= 5000 µm ÷ 25
= 200 µm (that is 0.2 mm)
Why it happens: the eyepiece cannot tell you a size on its own, because it magnifies the specimen and the ruler by exactly the same amount. But the field of view is a fixed real length for a given objective, so once it has been measured in millimetres it becomes a scale bar that stays valid for every slide viewed with that same objective. This is why step 2 must be done before the ruler is removed, and why the objective must not be changed afterwards.
Tip: the count must be along the diameter, in one straight line — not across the whole circle. Counting along a chord instead of the diameter would use too short a length and make the cells look bigger than they are.