Seeing the invisible

Look at two leaves on a faraway tree. They blur into one green patch. Your eye cannot tell two things apart if they are too close together. This is the eye's limit of resolution. Held about 25 cm away (the closest you can see clearly), two dots less than about 0.1 mm apart look like one dot. Most cells are smaller than that, so we need help to see them.

That help is a convex lens, or better, two of them together: an objective lens near the object and an eyepiece near your eye. In 1665, Robert Hooke looked at a thin slice of cork under a microscope he had designed himself (about 200 to 300 times magnification). He saw tiny box-like spaces and named them cells.

How big is a cell? Estimate it. Put a clear plastic ruler under the microscope and see how wide the circle you can see (the field of view) is. Say it is 5 mm. Since 1 mm = 1000 µm (micrometres), that is 5000 µm. Now count the cells lying across it: say 25. Each cell is about 5000 ÷ 25 = 200 µm wide.

How much bigger does it look? Multiply the two lenses: a 10X eyepiece × a 10X objective = 100X. So a 200 µm cell looks 100 times larger than it really is.

Your phone camera is a good way to remember the three things that make a microscope good:

• Resolution: how clear and sharp the picture is.
• Contrast: how different the bright and dark parts look.
• Magnification: how much bigger it makes things look.

An electron microscope uses a beam of electrons instead of light. It shows the parts inside a cell at the nanometre scale. A nanometre is one-billionth of a metre.