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Optical microscopes

Optical microscopes

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Which instrument allows us to see tiny organisms that cannot be seen with the naked eye?

Optical microscopes

If you ask someone what causes most diseases, they probably wouldn't have to think too hard about the answer: bacteria or viruses! These tiny organisms - microbes - attack the cells our bodies are made of, and this makes us sick! But this hasn’t always been obvious. It wasn’t until the second half of the 1600s that people realised the existence of bacteria or even cells. Why?

Simply because there was no way of seeing them! The smallest thing a person can see with the naked eye is about as thick as the tip of your hair. The invention of the optical microscope, however, allowed us to see the invisible... This is one of the first optical microscopes, built around 1590. It consists of a tube with two outward-curved lenses mounted at each end.

When you look at something through the tube, light reflected from the object enters through the first lens. This first lens is called the objective and it creates a magnified image. This image is further magnified by the second lens — the eyepiece. The final image that reaches the eye looks a few times bigger than the actual object. This microscope uses light and has more than one lens - it is a compound light microscope.

The design is quite basic and the image produced is blurry and dark. But the design and lenses improve over time, and the quality of images and magnification gets better. By the second half of the 1600s, microscopes can magnify images up to 270 times, enough to observe cells and bacteria for the first time. Today, optical microscopes are much more advanced, but the basic way they work remains the same. Let's take a look!

At the bottom of the microscope’s body there is a light source. It can be either a small lamp, or a mirror that allows you to direct the light. Above the light source there is the stage. Here you put the object you want to see — the specimen. The specimen needs to be really thin and placed on a transparent plate so that the light can come through and reach the objective.

Most microscopes have multiple objectives placed on a revolving wheel, sometimes called the nose piece. These objectives have lenses that provide different levels of magnification. The wheel allows you to switch between them. Just like the first microscopes, lenses in the objective magnify the image, which is further magnified by lenses in the eyepiece. But if a specimen isn’t at the right distance from the lens, the image will be blurry.

Luckily, there are focus knobs that allow us to adjust the distance with great precision, until the image is in focus. The size of the final image you see depends on the combination of lenses used in the objective and the eyepiece. If you multiply the level of magnification of the eyepiece and objective, you will obtain a number that describes the total magnification. For example, if you use an eyepiece that magnifies 10 times and an objective that magnifies 20 times, the total magnification will be equal to 10 times 20 — 200. The image will be 200 times bigger than the actual specimen.

The maximum total magnification of common microscopes — used in school labs or by amateurs — is around 400. But professional optical microscopes can magnify an image by up to 1500 times! Optical microscopes help us learn about microbes, diseases and the cells that make up our bodies. And as microscopes get better and more advanced, who knows what we might find under the microscope next...?