microscopes

Question 1

what type of microscopes were first developed

Answer 1

light microscopes
16th-17th century

Question 2

how does a light microscope work

Answer 2

• compound light microscope has 2 lenses
• objective lens produces a magnified image which is magnified again by the eyepiece lens
• objective/eyepiece lens configuration allows for much higher magnification and reduced chromatic aberration
• illumination is provided by a light underneath the sample

Question 3

sample preparation in light microscopy

Answer 3

dry mount
- solid specimens are viewed whole or cut with a sharp blade (sectioning). Cover slip is placed over specimen on the centre of the slide

wet mount
- specimens are suspended in liquid
- cover slip placed at an angle

squash slides
- wet mount is prepared
- lens tissue used to gently press down the cover slip
- damage to cover slip can be avoided by squashing the sample between 2 microscope slides

smear slides
- edge of a slide is used to smear the sample
- creates a thin even coating
- cover slip is placed

Question 4

purpose of stains

Answer 4

• increase contrast as cell cytosol and structures are often transparent
• which allows components to become visible so they can be identified

Question 5

what is magnification

Answer 5

how many times larger the image is than the actual size of the object being viewed

Question 6

resolution

Answer 6

• the ability to see individual objects as separate entities
• determines the amount of detail that can be seen

Question 7

what is resolution limited by

Answer 7

diffraction of light as it passes through the sample and lens

Question 8

how can resolution be increased

Answer 8

• using beams of electrons which have a wavelength 1000s of times shorter than light
• electron beams are still diffracted but the shorter wavelength means that individual beams can be much closer before they overlap

Question 9

equation for magnification

Answer 9

image= magnification x actual size

Question 10

why do microscopes need to be calibrated

Answer 10

the true magnification of the different lenses on a microscope can vary slightly from the magnification stated

Question 11

what is an eyepiece graticule

Answer 11

• glass disc marked with a fine scale of 1-100
• scale has no units and remains unchanged whichever objective lens is in place

Question 12

what is a stage micrometer

Answer 12

• microscope slide with a very accurate scale in micrometers engraved on it

Question 13

how to calibrate a microscope

Answer 13

• put the stage micrometer in place and the eyepiece graticule in place
• get the scale on the micrometer slide in clear focus
• align the micrometer scale with the scale in the eyepiece and take a reading from the two scales in order to calculate what 1 eyepiece graticule unit is equal to.
• Replace the micrometer slide with the sample slides and use the calibrated scale in the eyepiece to measure the sample

Question 14

when was the electron microscope invented

Answer 14

20th century

Question 15

how does electron microscopy work

Answer 15

• beam of electrons with a wavelength of less than 1nm illuminates the specimen
• more detail of cell ultrastructure can be seen because electrons have a much shorter wavelength than light waves

Question 16

disdvantages of electron microscopes

Answer 16

• expensive to buy and operate
• can only be used in a controlled environment
• large and needs to be installed
• complex sample preparation
• sample preparation often distorts material
• artefacts are produced
• vacuum is required
• black and white images produced (can be coloured digitally)
• specimens have to be dead

Question 17

advantages of electron microscopes

Answer 17

• over 500,000x magnification
• resolving power of transmission electron microscope is 0.5nm and scanning electron microscope is 3-10nm

Question 18

transmission electron microscope

Answer 18

• beam of electrons is transmitted through a specimen and focused to produce an image
• best resolution and resolving power of 0.5nm

Question 19

scanning electron microscope

Answer 19

• beam of electrons is sent across the surface of a specimen
• reflected electrons are collected
• resolving power is 3-10nm
• 3d image of surfaces are produced

Question 20

advantages of light microscopes

Answer 20

• inexpensive to buy and operate
• easily available
• small and portable
• can be used out in the field
• simple sample preparation
• sample preparation does not usually lead to distortion
• vacuum is not required
• specimens can be living or dead

Question 21

disadvantages of light microscopes

Answer 21

• only up to 2000x magnification
• resolving power is 200nm

Question 22

what is an artefact

Answer 22

• visible structural detail caused by processing the specimen and not a feature of the specimen
• appear in both light and electron microscopy
• bubbles trapped under cover slips are artefacts