When investigating the size of organisms and biological structures you will use a microscope of a specific magnification to produce an image
Photomicrographs are images obtained from a light microscope, these are used for specimens above 200 nm (a bacteria cell is about 1000 nm)
Electron micrographs are images obtained from electron microscopes, both scanning and transmission, these are used for specimens above 0.5 nm
Electron microscopes are useful for looking at organelles and biological molecules, eg. DNA can be seen replicating
To better understand the images we produce using microscopes we need to know the actual size of the specimen
Worked example: Calculating the actual size of a specimen
A scientist looks at a sample of red blood cells under a light microscope.
The eyepiece lens of the microscope has a magnification of x10 and an objective lens of x40 was used to view the blood cells. The scientist takes a photomicrograph of the blood cells, in which the average size of each cell is 3 mm.
What is the average size of the red blood cells in the sample? Give your answer in micrometres.
Eyepiece lens magnification: x10
Objective lens magnification: x40
Image size: 3 mm
Step 1: Calculate the total magnification of the specimen
eyepiece lens magnification x objective lens magnification
= total magnification
x10 x x40 = x400
Step 2: Calculate the image size in the units asked for (micrometres)
1 mm = 1000 μm
3 mm = 3000 μm
Step 3: Calculate the actual size of the red blood cell
Therefore, the average size of a red blood cell in this sample is 7.5 micrometres