Magnification and size

Last time

Before we start: last time, in Vertebrates, plants, viruses and keys, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

How big is a cell?

A drawing of a cell in your textbook might be ten centimetres across, but the real cell is a hundred times smaller than a millimetre. Magnification tells you how many times bigger the image is than the real object. One formula, and one unit change, gets every mark.

The formula

Magnification equals image size divided by actual size. Rearranged, the actual size equals the image size divided by the magnification. And the image size equals the actual size times the magnification. Magnification has no units. Write it with a times sign, for example times four hundred.

The triangle

Use the triangle. Image on top, actual size and magnification at the bottom. Cover the quantity you want, and the other two tell you whether to multiply or divide.

Units first

Cells are measured in micrometres. The symbol is the Greek letter mu, then m. There are one thousand micrometres in one millimetre. And ten millimetres in one centimetre, so one centimetre is ten thousand micrometres. Both lengths MUST be in the same unit before you divide.

Changing units

To change units: going to a smaller unit, multiply. Millimetres to micrometres, times one thousand. Going to a bigger unit, divide. Micrometres to millimetres, divide by one thousand.

Measuring the image

To find the image size, measure the drawing or photograph with a ruler, usually in millimetres. Measure along the line the question tells you, for example between two marked points. Then convert the measurement into the same unit as the actual size.