Cells: Magnification & Resolution of Microscopes

Question 1

What is magnification?

Answer 1

• Magnification is how much bigger the image is than the specimen

Question 2

Calculating Magnification

Answer 2

• Magnification = Size of image / Size of real object

- M = I / A

Question 3

What is resolution?

Answer 3

• Resolution is how detailed an image is
• More specifically, it is how well a microscope can distinguish between two points that are close together
• If a microscope can’t separate two objects, then increasing the magnification won’t help

Question 4

Optical (Light) Microscopes

Answer 4

• Uses light to form an image
• Maximum resolution is about 0.2μm due to the wavelength of light
• Maximum useful magnification is around x1500
• Staining can produce coloured images
• Specimens can be living or dead

Question 5

Electron Microscopes

Answer 5

• Uses electrons to form an image (electron beams have a shorter wavelength than light rays)
• Maximum resolution is about 0.0002μm - about 1000 times higher than optical microscopes
• Maximum useful magnification is about x1,500,000
• Produce black and white images, but the images can be coloured by a computer
• Electrons are absorbed by air so specimen is placed in a vacuum so is always dead

Question 6

Optical Microscopes vs Electron Microscopes

Answer 6

• Magnification

• Optical: Lower (max of x1500)
• Electron: Higher (max of x1,500,000)

• Resolution

• Optical: Lower (max of 0.2μm)
• Electron: Higher (max of 0.0002μm)

Question 7

Transmission Electron Microscopes

Answer 7

• TEMs use electromagnets to focus a beam of electrons, which is then transmitted through the specimen
• Denser parts of the specimen absorb more electrons, which makes them look darker in the produced image
• TEMs have high resolution
• The image produced is 2D and shows detailed images on the internal structure of cells and organelles

Question 8

Scanning Electron Microscopes

Answer 8

• SEMs scan a beam of electrons across the specimen
• The electrons are beamed onto the surface and the electrons are scattered in different ways depending on the contours
• The image produced shows the surface of the specimen and can be 3D
• SEMs are good because they can be used on thick specimens, but have a lower resolution than TEMs

Question 9

Advantages & Disadvantages of TEMs & SEMs

Answer 9

• TEMs

• Advantages:
• Gives high resolution images, so shows small objects
• Disadvantages:
• Can only be used on thin specimens
• Can only be used on dead specimens

• SEMs

• Advantages:
• Can be used on thick specimens
• Can produce 3D images
• Disadvantages:
• Give lower resolution than TEMs
• Can only be used on dead specimens

Question 10

Preparing Microscope Slides

Answer 10

• Start by pipetting a small drop of water onto the centre of the slide
• Use tweezers to place a thin section of your specimen on top of the water drop
• Add a drop of a stain - stains are used to highlight objects in a cell
• Add the coverslip
• Stand the slip upright, next to the water droplet and lower so it covers the slide
• Try not to get any air bubbles under there - they obstruct the view of the specimen

Question 11

Microscope Artefacts

Answer 11

• Artefacts are things you can see down the microscope that aren’t part of the cell or specimen that you’re looking at
• Can be dust, fingerprints, air bubbles, excess stain etc.
• Artefacts are caused during your preparation of the sample and shouldn’t be there
• Initially it was hard to distinguish between organelles and artefacts
• Organelles were seen repeatedly even when the specimens were prepared in different ways - artefacts were not