Chapter 9 — Microscope and Microscopy

1. What is a microscope?

A microscope allows us to observe objects too small to be resolved clearly by the naked eye.

The chapter traces microscopy from Robert Hooke’s observation of cork to modern light and electron microscopes.

The human eye can distinguish points separated by roughly $0.1\,mm$ under the conditions described in the chapter. Microscopes extend our ability to resolve much smaller structures.

2. History

Important milestones include:

• early magnifying glasses;
• Janssen compound microscope;
• Robert Hooke’s observation of cork;
• Leeuwenhoek’s observations of microorganisms;
• Ernst Abbe’s work on resolution;
• development of electron microscopy;
• phase-contrast microscopy.

3. Light microscopes

Light microscopes use:

• visible light;
• glass lenses.

Two basic types discussed are:

Simple microscope: one lens.

Compound microscope: multiple lens systems, mainly objective and eyepiece.

Advanced optical methods include:

• fluorescence microscopy;
• phase-contrast microscopy.

Phase contrast is especially useful for observing living cells without relying on chemical staining that could compromise the specimen.

4. Compound microscope

Important components include:

• illuminator/light source;
• condenser;
• stage;
• objective lenses;
• eyepiece;
• nosepiece;
• coarse adjustment;
• fine adjustment;
• diaphragm;
• arm;
• base.

The objective produces an enlarged image that becomes the input for the eyepiece, which further magnifies it.

5. Temporary and permanent mounts

A temporary mount is useful for short-term observation.

A permanent mount is prepared for long-term storage.

The chapter describes permanent mounting as involving:

fixation → staining → dehydration → mounting medium → coverslip

Materials such as Canada balsam or DPX can be used as mounting media.

6. Magnification

Total magnification:$$M=m_o\times m_e$$

where:

• $m_o$ = objective magnification;
• $m_e$ = eyepiece magnification.

Example:$$40X\times10X=400X$$

Actual size can be estimated from:$$\text{Real size}= \frac{\text{Image size}}{\text{Total magnification}}$$

7. Magnification vs resolution

These are not the same thing.

Magnification tells us how much larger the image appears.

Resolution tells us how well two closely spaced points can be distinguished.

A highly magnified but poorly resolved image may simply be a large blur.

The chapter gives approximate resolutions:

• human eye → $10^{-4}\,m$
• light microscope → $2\times10^{-7}\,m$
• electron microscope → $2\times10^{-10}\,m$

Thus, higher useful magnification requires adequate resolution.

8. Electron microscopes

Electron microscopes use electron beams rather than visible light.

Because electrons have a much shorter wavelength, they can provide much higher resolution.

TEM — Transmission Electron Microscope

• electrons pass through a very thin specimen;
• reveals internal structures;
• produces a 2D image;
• useful for structures such as mitochondria, ribosomes and viruses.

SEM — Scanning Electron Microscope

• scans the specimen surface;
• provides 3D-like surface images;
• useful for pollen grains, insect surfaces and microchips.

Electron microscopes require vacuum conditions and extensive specimen preparation, so they cannot normally be used to observe living cells.

Key comparison

Feature	Light	TEM	SEM
Source	Visible light	Electron beam	Electron beam
Main view	General specimens	Internal detail	Surface
Living cells	Possible	No	No
Image	Optical	2D	3D-like
Resolution	Lower	Very high	Very high

Chapter 9 — Microscope and Microscopy

The chapter covers the historical development of microscopes, simple and compound microscopes, magnification, resolution, and TEM/SEM.

Questions

A. Multiple Choice Questions

1. Robert Hooke is associated with the first use of the term:
a) Atom
b) Cell
c) Gene
d) Electron

2. A simple microscope uses:
a) One main lens
b) Two objective lenses
c) An electron beam
d) No lens

3. A compound microscope uses an objective lens and an:
a) Condenser only
b) Eyepiece
c) Electron gun
d) Aperture only

4. The objective lens is located:
a) Near the observer’s eye
b) Near the specimen
c) Inside the light source
d) Below the base

5. Resolution refers to the ability to:
a) Increase image brightness only
b) Distinguish closely spaced points
c) Increase specimen mass
d) Change the specimen’s colour

6. Total magnification of a compound microscope is obtained by:
a) Adding objective and eyepiece magnifications
b) Subtracting them
c) Multiplying them
d) Dividing objective by eyepiece

7. TEM is especially suited to studying:
a) Internal cellular ultrastructure
b) Only surface texture
c) Large landscapes
d) Sound waves

8. SEM is especially useful for studying:
a) Surface details
b) Internal atomic energy levels
c) Blood pressure
d) Temperature

B. Fill in the Blanks

9. Antony van Leeuwenhoek is widely regarded as the ______ of microscopy.

10. The lens nearest the specimen in a compound microscope is the ______ lens.

11. The lens through which the observer views the image is the ______ lens.

12. The ______ concentrates light on the specimen.

13. Total magnification equals objective magnification multiplied by ______ magnification.

14. TEM produces a mainly ______-dimensional image.

15. SEM produces a ______-like representation of specimen surfaces.

C. True or False

16. Greater magnification always guarantees greater resolution.

17. Light microscopes use visible light.

18. Electron microscopes use electron beams instead of visible light.

19. TEM is mainly used to study surface topography.

20. SEM is useful for examining specimen surfaces.

D. Short Answer

21. What is the difference between magnification and resolution?

22. What is the function of the objective lens?

23. Why are electron microscopes generally unable to observe living cells?

24. Differentiate between TEM and SEM.

E. Numerical Questions

25. A microscope has a $10X$ eyepiece and a $40X$ objective. Find the total magnification.

26. A cell produces an image $5mm$ long under $100X$ magnification. Calculate its actual length.

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Answers

A. MCQ Answers

1. b) Cell
2. a) One main lens
3. b) Eyepiece
4. b) Near the specimen
5. b) Distinguish closely spaced points
6. c) Multiplying them
7. a) Internal cellular ultrastructure
8. a) Surface details

B. Fill in the Blanks

9. Father
10. objective
11. eyepiece/ocular
12. condenser
13. eyepiece
14. two
15. three-dimensional

C. True/False

16. False
17. True
18. True
19. False
20. True

D. Answers

21. Magnification describes how much larger the image appears. Resolution describes the ability to distinguish two closely spaced points as separate.
22. The objective lens collects light/electrons from the specimen and forms the initial enlarged image.
23. Electron microscopy requires conditions such as vacuum and extensive specimen preparation that are incompatible with maintaining living cells.
24. TEM allows electrons to pass through very thin specimens and is mainly used to study internal structures. SEM scans specimen surfaces and provides detailed surface information with a three-dimensional appearance.

E. Numerical Answers

$$M=10\times40$$$$\boxed{400X}$$

$$\text{Actual size}=\frac{\text{Image size}}{\text{Magnification}}$$$$=\frac5{100}$$$$\boxed{0.05mm}$$