• 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