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Seeing is believing

An invisible world

Imagine what we would know about the world if we could see things that the human eye can't see?

What is the smallest thing we can see with the naked eye?

The closest a human eye can focus on is about 20cm. We cannot see anything under 0.05mm or 50 µm. As a typical animal cell is only about 10 µm, we didn't understand the existence of the basic building block of life until the invention of the microscope.

The big invention that put the very small on the map

The first person to use a microscope was Robert Hooke. He was a famous scientist during the 17th century. Hooke published a book called Micrographia which contained many stunning drawings of fleas, cheese mites and many other tiny creepy crawlies. These were a revelation to people of the 17th century who had never seen anything like it before.

Creepy crawlies

The first really good microscope

The first person to make a really good microscope was Antony van Leeuwenhoek. He sold material and used magnifying glasses to check the quality of the weave in the cloth. He taught himself how to grind magnifying lenses. He became so good at this that he made a really good microscope that could magnify at over 200X. This is a pretty fair performance for a school microscope today and this was well over 300 years ago!

Work of Antony van Leeuwenhoek

Where did the term cell come from?

Hooke examined cork under his microscope and saw regular-shaped plant cells. He called them cells because he thought they looked like the little rooms that monks lived in.

How small can you see?

The absolute limit for observation of an ordinary microscope is set by the wavelength of light. About the smallest thing that can be seen is about 200nm or 200 x 10-9m. It doesn’t matter how much magnification you use, you will never see smaller than this because the light waves just move past such a small obstacle without being affected. It’s a bit like watching waves on the beach washing over tiny pebbles. This means that a light microscope can never magnify more than about x1000. To get more usable magnification than this we need to use shorter wavelengths.

That's where the electron microscope comes in!

Electron microscopes use beams of electrons. If you can get electrons moving fast enough they stop behaving like little lumps of stuff and start behaving like wavy things. This is called wave particle duality and is an idea that goes very deep into all of science. The faster something moves the wavier it gets. If you take an electron and accelerate it up to close to the speed of light, it gets very wavy indeed. It fact, it has a wavelength about the same as an x-ray. This is about a million times shorter than visible light so you can see about a million times more detail.

How does an electron microscope work?

Fast moving electrons are bent and focussed using magnetic lenses in exactly the same way Van Leeuwenhoek used his glass lens. The picture is seen on a fluorescent screen where the electrons give up some of their energy as light. There are two main types of electron microscope.

  • The transmission electron microscope

The transmission microscope passes a beam of electrons through a specimen. It can produce highly magnified images up to x 50 million! It does have disadvantages, it can only image very thin slices of material and so can’t see things in three dimensions.

  • The scanning electron microscope.

Scanning microscopes can’t do such high magnifications, but they can see really complex surfaces. This is what produces all the stunning images of bugs! They scan a tiny beam of electrons across the surface building up the image a tiny piece at a time.

What can we see with the most powerful microscopes today?

This is the most magnified image ever produced. It shows a single molecule, this is pentacene.

The most magnified image ever produced

The big questions:

Why have microscopes?

How small can we see?

Why is there a limit to magnification?

How does an electron microscope work?

What benefits have been gained from using microscope?

What uses may microscopes have in hospitals?