Microscopes

• Light microscopes are better because they use light to form images, living samples can be used and they are relatively cheap.
• Light microscopes have a low resolution and low magnification.
• Electron microscopes are useful because they use a beam of electrons to form images, have a high magnification and have a high resolution.
• Electron microscopes are expensive and samples cannot be living.
• The aim of the microscopes required practical is to produce a slide of specimen and use a microscope to observe their cells and their organelles.
• The equipment needed for the required practical is a light microscope, slide, cover slip, iodine, paper towel, onion, tweezers, mounted needle and a knife.
• In order to prepare a slide, add a drop of water to the middle of a clean slide. Cut up an onion and separate it into layers. Use tweezers to peel off some of the epidermal tissue from the bottom of one of the layers. Using tweezers again, place the epidermal tissue into the water on the slide. Add iodine solution to stain objects on the inside of the cell. Place a coverslip on top by tilting it down from one side to avoid air bubbles. Carefully wipe away excess liquid with a paper towel.
• To observe the specimen, clip the prepared slide onto the stage. Select the lowest powered objective lens. Use the coarse adjustment knob to move the stage up to just below the objective lens. Look down the eyepiece lens and use the coarse adjustment knob to move the stage down until the image is roughly in focus. Adjust the fine focusing knob until you get a clear image. If you need to see the slide with a greater magnification, swap to a higher powered objective lens and refocus.
• total magnification = objective lens x eyepiece lens
• image size = actual size X magnification
• An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.
• magnification = size of image

  size of real object