Microscopes

• How are electron microscopes different to light microscopes? (5)

  • electron microscopes use beams of electrons instead of light
  • cannot be used to view living samples
  • much more expensive
  • have a much higher magnification and resolution
  • allow you to see sub-cellular structures that can't be seen with a light microscope
• What microscopes to schools typically have?

  light microscopes
• What are the two types of electron microscopes?

  Transmission Electron Microscope (TEM) & Scanning Electron Microscope (SEM)
• What kind of images do Transmission Electron Microscopes give?

  2D images
• What kind of images do Scanning Electron Microscopes give?

  3D images but lower magnification
• What is the resolving power of a Light Microscope?

  200nm
• What is resolving power?

  The limit up to which two small objects are still seen as separate entities.
• What is the equation for magnification?

  Magnification = Image size / Actual size

  
• 1km = 1000m

  1m = 100cm

  1cm = 10mm

  1mm = 1000μm (micrometres)

  1μm = 1000nm (nanometres)
• What is the most common form of animal cells when using microscopes?

  cheek cells
• What is the most common form of plant cells when using microscopes?

  onion cells
• What stains are used to highlight the structures within a cheek cell?

  methylene blue
• What stains are used to highlight the structures within an onion cell?

  iodine
• How to view an image in focus. (6)

  1. Clip the slide carefully onto the stage
  2. Ensure the lowest-powered objective lens is over the slide.
  3. Use the coarse adjustment knob to bring the stage up just below the lens.
  4. Look down the eyepiece and gradually move the stage downwards using the coarse adjustment knob. Stop when the image is roughly in focus.
  5. To bring the image into focus, adjust the fine-adjustment knob until a clear image is obtained.
  6. To observe the image with a higher magnification, change the objective lens to a higher power and readjust the stage using the coarse and fine adjustment knobs.
• How to prepare an onion cell on a slide. (5)

  1. Peel a thin layer of onion tissue/membrane
  2. Place on a slide carefully ensuring to keep it in one layer
  3. Place one/two drops of iodine solution on the onion tissue
  4. Cover with a cover slip & press down carefully to remove air bubbles
  5. Blot wim a tissue to remove excess iodine solution.
• How to prepare a cheek cell on a slide. (5)

  1. Rub a clean cotton swab on inside of cheek
  2. Dab or roll cells from the swab onto a slide.
  3. Place a drop of methylene blue stain onto the slide.
  4. Place a cover slip on top of cells, and press down carefully to remove air bubbles
  5. Blot away excess of stain
• Hazards of preparing a sample

  • Taking care when handling glass e.g. cover slip - Tell a teacher
  • Hold solutions by the bottle to prevent spills
  • Wear goggles when handling iodine
  • Wash hands if methylene blue gets on your skin
  • Put cotton buds & slides into disinfectant to prevent cross contamination