Required Practical: Using a light Microscope

Below is a diagram of a light microscope, otherwise known as an optical microscope. Although. some optical microscopes may look slightly different.

Diagram of a light microscope

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 course focus and fine focusing knobs.

Making slides

If you want 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.

Slide of cells that are viewed under a microscope


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 one object is in focus, the other objective lenses are also in focus as you move from one x4 to x10 etc. You may need to make slight adjustments when changing objective lenses, but this is not necessary when you start with the x4 lens.