Microscopes

• The difference between magnification and resolution

  Magnification - how much bigger the image of a sample is compared to the real size,

  measured by 𝐌𝐚𝐠𝐧𝐢𝐟��𝐜𝐚𝐭𝐢𝐨𝐧 =

  𝐒𝐢𝐳𝐞 𝐨𝐟 𝐢𝐦𝐚𝐠𝐞

  𝐒𝐢𝐳𝐞 𝐨�� 𝐫𝐞𝐚𝐥 𝐨𝐛𝐣𝐞𝐜𝐭

  • Resolution - how well distinguished an image is between 2 points; shows amount of detail;

  limited by wavelength of radiation used e.g. light
• Measuring the size of an object viewed with an optical microscope

  Line up eyepiece graticule with stage

  micrometer

  Use stage micrometer to calculate the size

  of divisions on eyepiece graticule at a

  particular magnification

  Take the micrometer away and use the

  graticule to measure how many divisions

  make up the object

  Calculate the size of the object by multiplying the number of divisions by the size of

  division

  • Recalibrate eyepiece graticule at different magnifications

  
• Preparing a ‘temporary mount’ of a specimen on a slide

  Use tweezers to place a thin section of specimen e.g. tissue on a water drop on a

  microscope slide

  Add a drop of a stain e.g. iodine in potassium iodide solution used to stain starch grains in

  plant cells

  • Add a cover slip by carefully tilting and lowering it, trying not to get any air bubbles
• Optical microscope

  • Use light to form a 2D image
  • Visible light longer wavelength so lower resolution 200nm
  • Low magnification x1500

  bad- 2D image

  bad- Only used on thin specimens bad- Low resolution; can't see internal structures of organelles or organelles smaller than 200nm e.g. ribosomes

  • Low magnification

  good- Can see living organisms.
• Scanning electron microscope

  Use electrons to form a 3D

  image

  Beams of electrons scan surface, knocking off electrons from the specimen, which are gathered in a cathode ray tube to form an image

  Electrons shorter wavelength (so higher resolution 0.2nm)

  • High magnification x1500000

  — Vacuum; can't see living organisms

  — Lower resolution than TEM

  + 3D image

  + High resolution; can see internal structures of organelles

  • High magnification

  good- Used on thick specimens
• Transmission electron microscope

  -

  Use electrons to form a 2D image

  Electromagnets focus beam of electrons onto specimen, transmitted, more dense =

  more absorbed = darker

  appearance

  Electrons shorter wavelength (so higher resolution 0.2nm)

  • High magnification x1500000—2D image

  —Only used on thin specimen

  —Vacuum; can't see living organisms

  good- High resolution; see internal structures of organelles

  good- High magnification