2.1.1 - Microscopes

Question 1

What is an electron micrograph?

Answer 1

a photograph of an image seen using an electron microscope

Question 2

What is magnification?

Answer 2

the number of times larger an image appears, compared with the size of an abject

Question 3

What is resolution?

Answer 3

the clarity of an image; the higher the resolution, the clearer the imahe

Question 4

Advantages of Optical Microscopes

Answer 4

cheap
easy to use
portable - use in the lab and field
can study whole living specimens

Question 5

What is the magnification of optical microscopes?

Answer 5

x1500 (x2000 for some)

Question 6

What type of light do optical microscopes use?

Answer 6

use visible light
- part of the electromagnetic spectrum with a wavelength between

400nm and 700nm

Question 7

What is the wavelength range of optical microscopes?

Answer 7

the wavelength of visible light ranges from

400nm to 700 nm

so structures closer together than 200nm (0.2 um) will appear as one object.

Question 8

What are features of ribosomes?

Answer 8

ribosomes are very small, non membrane bound cell organelles of about 20nm diameter
- so they cannot be examined using a light microscope

Question 9

How do you calculate magnification?

Answer 9

total magnification =

magnifying power of the objective lense x
magnifying power if the eyepiece lens

Question 10

What is another name for laser scanning microscopes?

Answer 10

confocal microscopes

Question 11

What type of light do laser scanning microscopes use? How do they do this?

Answer 11

laser light it used
- it scans an object point by point
pixel information is assembled by computer into one image
displayed on a computer screen

Question 12

What is the resolution and contrast of laser scanning microscopes like?

Answer 12

images are high resolution and show high contrast

Question 13

What is depth selectivity?

Answer 13

LSMs can focus on structures at different depths within a specimen
can be used to clearly observe whole living specimens as well as cells

Question 14

Where are laser scanning microscopes used?

Answer 14

• the medical profession
  e.g. to observe fungal filaments within the cornea of the eye with an infection
  this gives:
• quick diagnosis
• earlier and more effective treatment

-different branches of biological research

Question 15

How do electron microscopes work?

Answer 15

they use a beam of fast-travelling electrons
- electrons are fired from a cathode and focused by magnets rather than glass lenses on to a screen or photographic plate

Question 16

What is the wavelength of electron microscopes?

Answer 16

0.004nm

Question 17

What is the resolution of electron microscopes like?

Answer 17

greater resolution than optical microscopes

can be used to give clear and highly magnified images

Question 18

How is the specimen prepared for transmission electron microscopes?

Answer 18

the specimen has to be chemically fixed by being dehydrated and stained

Question 19

How does the image appear in transmission electron microscopes?

Answer 19

the beam of electrons pass through the specimen, which is stained with metal salts
some electrons pass through and are focused on the screen or photographic plate
electrons form a 2D black greyscale image

Question 20

What magnification can transmission electron microscopes produce?

Answer 20

a magnification of up to 2 million times

Question 21

What are scanning electron microscopes?

How do they work?

Answer 21

• electrons do not pass through specimen in whole
• causes secondary electrons to bounce off the specimen’s surface and be focused on to a screen
• specimen is placed in a vacuum and is often coated with a fine film or metal
• gives 3D image
• greyscale image produced but computer programmes can add false colour

Question 22

What is the magnification of scanning electron microscopes?

Answer 22

x15 to 200,000

Question 23

Disadvantages of electron microscopes

Answer 23

• large and expensive

- need a great deal of skill and training to use

Question 24

Disadvantages of scanning electron microscopes

Answer 24

• specimen, even whole ones have to be dead
  as they are viewed while in a vacuum
• metallic salt stains may be potentially hazardous to the user