How do microscopes reveal tiny details?

1. Nia peels a thin, see-through skin from an onion. To her eyes it looks plain and smooth. Its parts are far too small to see. She needs a microscope.
2. The skin goes on a glass slide, on the flat stage. A lamp underneath shines light straight up through the thin sample and into the lens above it.
3. Just above the slide sits a small, strongly curved lens called the objective. It bends the light from each point of the sample so the rays spread into a bigger picture inside the tube.
4. At the top of the tube is a second lens, the eyepiece. It works like a magnifying glass on the picture the first lens made, so the picture gets bigger again.
5. At first the picture is a blur. Nia turns the focus knob, which moves the stage up and down a tiny bit. When the sample is exactly the right distance from the lens, the picture snaps sharp.
6. Now Nia can see it: the onion skin is built from tiny boxes packed like bricks. They are cells, and the dark dot in each one is its control center, the nucleus.

Tap each part. Follow the light from the lamp at the bottom all the way up to your eye.

• Lamp. The lamp in the base shines light upward through the hole in the stage. Without it the thin sample would be too dark to see.
• Diaphragm. Under the stage is a ring with a hole that can open wide or close small. It controls how much light reaches the sample.
• Slide and stage. The sample sits on a strip of glass called a slide. The slide rests on the flat stage, over a hole that lets the light through, and clips hold it still.
• Objective lenses. Three objective lenses sit on a wheel that turns. The short one makes things a little bigger. The long one makes them much bigger. You click the one you want into place.
• Body tube. The tube holds the two lenses exactly the right distance apart and keeps stray light out. Inside it, the first lens makes its bigger picture.
• Eyepiece. You look into the eyepiece at the top. It is a magnifying lens that makes the picture from the first lens about ten times bigger again.
• Focus knobs. The big knob moves the stage up and down quickly to find the picture. The small knob moves it very slowly to make the picture perfectly sharp.

Turn the focus knob until the picture is sharp. Move the slide to look around. Then switch to a stronger lens. What changes?

Experiment: You switch from the weakest lens to the strongest lens. What happens to the picture?

1. Use the weakest lens and turn the focus knob until the picture is sharp.
2. Move the slide so something interesting is in the middle.
3. Switch to the strongest lens and focus again. Compare what you see.

What we found: A stronger lens shows a smaller part of the sample, made much bigger. That is why you find things with the weak lens first and then zoom in. The strong lens also needs more exact focus.

Turn on JavaScript to try it yourself.

Find the water bear, get it in the middle, then zoom in and take a sharp photo. The stronger the lens, the more stars.

A lens is a piece of glass that is thicker in the middle than at the edges. When light passes through it, the light bends. Light from one tiny point of a sample spreads out, goes through the lens and is bent so that it seems to come from a much bigger picture. A magnifying glass is one lens. It can make things look about two to ten times bigger.

A microscope uses two lenses, one after the other. The first one, the objective, sits very close to the sample and makes a bigger picture inside the tube. The second one, the eyepiece, magnifies that picture again. To find the total, you multiply: a 40 times objective and a 10 times eyepiece make things look 400 times bigger. A hair would look as wide as a tree trunk.

There is a price for zooming in. A stronger lens shows a smaller patch of the sample, so it is easy to lose what you were looking at. The picture gets dimmer, because the same light is spread over a bigger picture. And the focus must be more exact. That is why scientists start with the weakest lens, find the thing, put it in the middle and only then switch up.

Our comparison with two magnifying glasses is not perfect. In a real microscope the first lens flips the picture upside down and back to front. If you push the slide to the left, the picture moves to the right!

Key words

Lens

A curved piece of glass or plastic that bends light.

Objective

The lens closest to the sample. It makes the first, enlarged image inside the tube.

Eyepiece

The lens you look into. It magnifies the image made by the objective.

Magnification

How many times bigger something looks.

Field of view

The width of the patch of sample you can see at one time.

Resolution

How close two tiny dots can be and still look like two dots, not one.

Cell

The tiny building block that all living things are made of.

Check your thinking

1. What does a lens do to light to make things look bigger?

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   It bends the light. A lens bends light so the rays spread out and seem to come from a bigger picture.

2. The picture is blurry. Which part do you turn?

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   The focus knob. The focus knob moves the stage until the sample is exactly the right distance from the lens.

3. A microscope has a 10 times eyepiece. You click the 40 times objective into place. How much bigger do things look?

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   400 times. You multiply the two lenses: 40 times 10 is 400.

4. Why do scientists look for something with the weakest lens first?

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   It shows the biggest patch of the sample, so things are easy to find. The weak lens shows the most of the slide. Once the thing is in the middle, you can zoom in without losing it.

5. Why can a light microscope not show details much smaller than 0.2 micrometers, however strong its lenses are?

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   Light is a wave and spreads out, so each point becomes a small blur. Diffraction blurs every point into a spot about half a wavelength wide. More magnification only enlarges that blur.

6. Why must a sample for this kind of microscope be very thin?

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   So light can shine through it. The lamp is under the sample. Light has to pass up through it to reach the lenses.