A student examines cells from a moss leaf, first with a light microscope and then, for one chloroplast within a cell, with an electron microscope. (a) In the light micrograph, one moss leaf cell appears 3.6 mm long. This micrograph was taken at a magnification of ×120. Calculate…

IGCSE Biology 0610: Calculating Actual Size and Magnification With Micrometres

Part (a): Calculating an actual size from an image size and magnification

The magnification formula can be rearranged to find an actual size when the image size and the magnification are both known:

actual size = image size ÷ magnification

For the moss leaf cell, the image length is 3.6 mm and the magnification is ×120:

3.6 mm ÷ 120 = 0.03 mm

So the actual length of the moss leaf cell is 0.03 mm.

Part (b): Converting micrometres to millimetres

There are 1000 micrometres (μm) in every millimetre (mm), so to convert from μm to mm, divide by 1000:

5 μm ÷ 1000 = 0.005 mm

So the actual diameter of the chloroplast is 0.005 mm.

Part (c): Calculating a magnification using mixed units

The magnification formula is:

magnification = image size ÷ actual size

Before dividing, both lengths must be in the same unit. The image diameter is given in mm (0.4 mm) and the actual diameter is given in μm (5 μm), so one must be converted first.

Using the value converted in part (b), the actual diameter is 0.005 mm, so both lengths are now in mm:

0.4 mm ÷ 0.005 mm = 80

(Checking the other way: converting the image diameter to μm gives 0.4 mm × 1000 = 400 μm, and 400 μm ÷ 5 μm = 80. The same answer, confirming the calculation.)

So the magnification of the electron micrograph is ×80.

Final answers

(a) Actual length of moss leaf cell = 0.03 mm
(b) Actual diameter of chloroplast = 0.005 mm
(c) Magnification of electron micrograph = ×80