### KS3 Biology

Cells and Organisation
Health
Material Cycles and Energy
Relationships in an Ecosystem

# Microscopes

We can break down the word microscope. It includes the prefix ‘micro‘, which means small, and the suffix ‘scope‘, which means to look at something carefully. Therefore, a microscope is an instrument we use to magnify small objects.

We typically want to see small things that cannot be seen with the naked eye. For example:

• Bacteria
• Fungi
• Microscopic animals (for example dust mites)

## The Light Microscope

In a light microscope, a series of lenses is used 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. Light passes through the eyepiece lens and then enters the eye.

### 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.

### Focusing

The focus knobs are used to sharpen 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.

Microscopes usually have three or four objective lenses mounted on a turret that can be rotated. It is best to start off with the objective lens with the lowest magnification, which is usually x4.

The x4 objective lens has the lowest magnification, so it has the largest field of view. This gives a broader view of the sample, helping you decide which part of it you want to examine more closely.

## Calculating the Magnification of a Microscope

To work out the total magnification of a microscope, the equation is:

So, if the eyepiece magnification is ×10 and the objective lens magnification is ×60, then the total magnification is:

10 × 60 = ×600

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