Required Practical: Using a light Microscope

Below is a diagram of a light microscope, otherwise known as an optical microscope.

A detailed diagram of a microscope with labelled parts. The labelled components include the Eyepiece/Ocular Lens, Diopter Adjustment, Revolving Nose Piece, Arm, Objective Lens, Slide Holder, Slide, Fine Focus, Coarse Focus, Stage, Condenser, Iris Diaphragm, Light Source, Base, and an On/Off Switch. Arrows and lines connect each label to the corresponding part of the microscope.

Although some light microscopes may look slightly different, they typically share the same structure.

In a light microscope, a series of lenses are used to produce a magnified image of an object.

  • At the centre of the microscope, there is a stage, which is where we place the microscope slide. A slide holder or a clip will hold it in place.
  • There’s a light source below the stage that passes light up, through the microscope slide.
  • Above the stage, there are a set of lenses, which we call objective lenses. Most microscopes have three different objective lenses with various magnification powers, with the most common being x4, x10, x40 and x100.
  • At the top of the microscope is the eyepiece, otherwise known as the ocular lens, which has a magnification of x10.

1. Place the slide onto the stage.

2. Turn on the microscope, so light passes through the slide.

3. Select the lowest powered objective lens, which is usually x4.

  • This lens is called the scanning objective lens

4. Look down the eyepiece lens and turn the coarse focus knob, to bring the image roughly into focus.

  • This increases the distance between the objective lens and the slide

5. Adjust the fine focusing knob, to bring the image into focus, producing a clear image.

6. To observe the sample with higher magnification, move the objective lenses, so that an objective lens with a higher magnification power clicks into place.

7. After switching to a different objective lens, you can readjust the focus by using the coarse focus and fine-focusing knobs.

Making slides

Let’s explore how to prepare slides to look at cells under a microscope.

To look at cells under a microscope, spread a very small sample of cells on a glass slide. Then add a few drops of dye, which stains the cells. After this, place a cover on top of the slide, so that the sample can be viewed.

A close-up microscopic view of plant cells, displaying a hexagonal pattern. Each cell is surrounded by a green wall and contains clear cytoplasm with tiny dark spots.


The focusing knobs are used to focus the image. The safest way to do this is by using the knobs to move the stage downwards, rather than upwards. When you focus upwards, there is a chance of the objective lens and slide colliding.

It is best to start with the objective lens that has the lowest magnification, which is usually the x4. The x4 objective lens has the lowest magnification, so it has the largest field of view. This allows you to see more of the sample and it allows you to observe which part of the sample you want to observe.

With most microscopes, if an object is in focus with one objective lens, it will likely remain in focus when switching to a different objective lens, like moving from x4 to x10. However, slight adjustments may be necessary.

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