Calculating Actual Size
- 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.
Known values:
- 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
