Cells are the basic building blocks of all animals and plants. They are so small, you need to use a light microscope to see them.

The light microscope

A modern microscope with two eyepiece lenses
A modern microscope with two eyepiece lenses

A light microscope uses a series of lenses to produce a magnified image of an object:

  1. the object is placed on a rectangular glass slide
  2. the slide is placed on a stage with a light source below
  3. light shines through the object and into the objective lens
  4. the light passes through the eyepiece lens and from there into your eye

You can focus the image using one or more focusing knobs. It is safest to focus by using the knobs to move the stage downwards, rather than upwards. There is a chance of the objective lens and slide colliding if you focus upwards.

Microscopes often have three or four objective lenses on a turret that you can turn. It is wise to observe an object using the lowest magnification lens first. You may need to adjust the focus and the amount of light as you move to higher magnifications.

Microscope view of plant cells
Plant cells seen through a light microscope

Making a slide

Onion cells are easy to see using a light microscope. Here is a typical method for preparing a slide of onion cells:

  1. cut open an onion
  2. use forceps to peel a thin layer from the inside
  3. spread out the layer on a microscope slide
  4. add a drop of iodine solution to the layer
  5. carefully place a cover slip over the layer
Onion cells with iodine stains, allowing the nuclei to be visible
Stains like iodine make features such as the nucleus easier to see

Observing cells

When you observe cells, it is usual to make a drawing of what you see. Very often there is so much to see that you can only aim to draw part of it:

  • use pencil rather than pen or colours
  • outline the features as accurately as you can
  • use as little shading as possible
  • label your drawing with the name of the sample and the total magnification you used
Epithelial (cheek) cells
Cheek cells stained with methylene blue

Total magnification

The magnification of each lens is shown next to the lens:

total magnification = eyepiece lens magnification × objective lens magnification

For example, if the eyepiece magnification is ×10 and the objective lens magnification is ×40:

total magnification = 10 x 40 = ×400 (400 times)