Unit Conversions

Cambridge International AS and A Level
Second Edition
Biology Mary Jones
© Mary Jones 2015
ISBN 978-1-4718-2887-4

Hodder Education, an Hachette UK company,
Carmelite House,
50 Victoria Embankment,
 London EC4Y 0DZ

Cells are the basic units from which living organisms are made. Most cells are very small, and their structures can only be seen by using a microscope.


Light rays pass through the specimen on a slide and are focused by an objective lens and an eyepiece lens. This produces a magnified image of the specimen on the retina of your eye. Alternatively, the image can be projected onto a screen, or recorded by a camera.


An electron microscope uses beams of electrons rather than light rays. The specimen has to be very thin and must be placed in a vacuum, to allow electrons to pass through it. The electrons are focused onto a screen, or onto photographic film, where they form a magnified image of the specimen.


Magnification and resolution

 Magnification can be defined as: magnification = size of image actual size of object This can be rearranged to: size of object = size of image magnification There is no limit to the amount you can magnify an image. However, the amount of useful magnification depends on the resolution of the microscope.

This is the ability of the microscope to distinguish two objects as separate from one another. The smaller the objects that can be distinguished, the higher the resolution.  Resolution is determined by the wavelength of the rays that are being used to view the specimen. The wavelength of a beam of electrons is much smaller than the wavelength of light.


An electron microscope can therefore distinguish between much smaller objects than a light microscope — in other words, an electron microscope has a much higher resolution than a light microscope. We can therefore see much more fine detail of a cell using an electron microscope than using a light microscope. As cells are very small, we have to use units much smaller than millimetres to measure them.

 These units are micrometres, µm, and nanometres, nm.

1 mm = 1 × 10⁻³ m

 1 µm = 1 × 10⁻⁶ m

 1 nm = 1 × 10⁻⁹ m 

To change mm into µm, multiply by 1000.