9.1
What is a Microscope?
Microscopes magnify tiny specimens that cannot be resolved clearly by the naked eye.
Microscope and Microscopy
Understand how microscopes reveal structures beyond the resolving power of the naked eye—from Hooke's cork cells to modern light, phase-contrast, fluorescence, TEM and SEM microscopy.
9.2
History of Microscopes
Trace microscopy from early magnifying glasses to electron and phase-contrast microscopes.
9.3
How a Microscope Works
Study magnification, resolution, contrast, lenses and the working of compound microscopes.
9.4
Microscopy Skills
Understand temporary and permanent mounts, focusing and quantitative magnification.
9.5
Types of Microscopes
Compare light microscopes, TEM and SEM using magnification, resolution and sample requirements.
9.6–9.7
Developments, Limits & Uses
Explore digital and super-resolution microscopy, limitations and real-world applications.
Chapter Roadmap
See the limit.
The human eye cannot resolve points that are too close together. At about 25 cm, the chapter gives a limit of about 0.1 mm (100 µm).
The human eye cannot resolve points that are too close together. At about 25 cm, the chapter gives a limit of about 0.1 mm (100 µm).
Magnify and resolve.
A microscope enlarges an image, but useful microscopy also depends on resolution and contrast.
A microscope enlarges an image, but useful microscopy also depends on resolution and contrast.
Understand the compound microscope.
The objective forms the first enlarged real image; the eyepiece further magnifies it for the observer.
The objective forms the first enlarged real image; the eyepiece further magnifies it for the observer.
Compare technologies.
Light microscopes use visible light and glass lenses, while electron microscopes use electron beams and electromagnetic lenses.
Light microscopes use visible light and glass lenses, while electron microscopes use electron beams and electromagnetic lenses.
Choose the right microscope.
The specimen and the purpose of observation determine the suitable microscope.
The specimen and the purpose of observation determine the suitable microscope.
9.1 What is a Microscope?
A microscope is a special instrument used to observe tiny living organisms or their parts that cannot be seen through the naked eye by magnifying them. Examples include onion cells, cheek cells, bacteria and dust particles.
Microscope: The word is derived from micro meaning
small and skopion meaning “means of viewing”.
Limit of resolution of the human eye
The ability of the eye to see two very close objects as separate and distinct is related to its resolution. At the near point of the eye, about 25 cm, two points separated by approximately 0.1 mm (100 µm) can be observed as distinct according to the chapter. Points closer than this may appear as a single point.
Figure 9.1: Compound Microscope Two-Stage Optical Magnification: Objective forms magnified real intermediate image, which is further enlarged by eyepiece.
Early microscopy
In 1665, Robert Hooke observed thin slices of cork under a microscope he had developed. He saw tiny hexagonal box-like spaces resembling honeycomb and called them cells. Around the same time, Antony van Leeuwenhoek made tiny, powerful lenses and observed “animalcules”, now known as bacteria and protozoa.
Key Questions
- What is a microscope used for?
- What is meant by the limit of resolution?
- Why is a cell generally invisible to the unaided eye?
- Who coined the term “cell” after observing cork?
9.2 A Quick Historical Journey of Microscopes
13th–15th century
Simple magnifying glasses were used by spectacle makers.
Simple magnifying glasses were used by spectacle makers.
1590 — Hans and Zacharias Janssen
The Dutch father-and-son spectacle makers developed an early compound microscope by combining two lenses in a tube.
The Dutch father-and-son spectacle makers developed an early compound microscope by combining two lenses in a tube.
1665 — Robert Hooke
Observed cork and coined the term “cell”; published the findings in Micrographia.
Observed cork and coined the term “cell”; published the findings in Micrographia.
1670s — Antony van Leeuwenhoek
Used a simple single-lens microscope capable of magnifying up to about 300× and observed bacteria and protozoa. The chapter calls him the Father of Microscopy.
Used a simple single-lens microscope capable of magnifying up to about 300× and observed bacteria and protozoa. The chapter calls him the Father of Microscopy.
1878 — Ernst Abbe
Postulated a mathematical theory linking resolution to wavelength.
Postulated a mathematical theory linking resolution to wavelength.
19th–20th century
Improved lenses and illumination advanced compound light microscopes.
Improved lenses and illumination advanced compound light microscopes.
1930s onwards
Electron microscopes such as TEM and SEM enabled observation of viruses, organelles and cell surfaces.
Electron microscopes such as TEM and SEM enabled observation of viruses, organelles and cell surfaces.
1938 — Ernst Ruska
Developed the first electron microscope, using electrons as the illumination source.
Developed the first electron microscope, using electrons as the illumination source.
1953 — Frits Zernike
Received the Nobel Prize in Physics for inventing and demonstrating the phase-contrast microscope.
Received the Nobel Prize in Physics for inventing and demonstrating the phase-contrast microscope.
Historical pattern: microscopy progressed from simple
magnification toward improved resolution, contrast and specialised ways of imaging specimens.
9.3 How Does a Microscope Work?
Important parameters in microscopy include resolution, contrast and magnification. The operating principle varies with the type of microscope, but microscopes use systems of lenses or electromagnetic fields to produce enlarged, detailed images of specimens that cannot be clearly seen with the naked eye.
9.3.1 Types of Light Microscope
Simple microscopeUses a single lens to magnify an object,
similar to a magnifying glass.
Dissecting microscopeGiven in the chapter as an example of
a simple microscope used for 3D viewing of small objects.
Compound microscopeUses at least two sets of lenses:
objective and eyepiece.
Fluorescence microscopeUses high-intensity light to excite
specialised dyes so selected structures glow.
Phase-contrast microscopeEnhances contrast without chemical
staining and is useful for viewing living cells.
Advanced optical microscopyUses specialised optical
techniques to reveal cellular structures that ordinary viewing may not show clearly.
9.3.2 Parts of a Compound Microscope
| Part | Role |
|---|---|
| Light source | Provides illumination, using an LED or halogen lamp; a mirror may also reflect light in some arrangements. |
| Condenser lens | Focuses and concentrates light on the specimen to optimise numerical aperture and contrast. |
| Specimen stage | Holds the slide containing the specimen. |
| Objective lens | Primary magnifying lens near the specimen; examples include 4×, 10×, 40× and 100×. |
| Eyepiece / ocular lens | Further magnifies the image already formed by the objective. |
| Body tube | Hollow tube with the eyepiece and objectives fitted at its ends. |
| Revolving nosepiece | Holds the objective lenses and allows switching between them. |
| Stage clips / mechanical stage | Hold the slide in position. |
| Substage diaphragm | Controls the amount of light transmitted to the specimen. |
| Coarse adjustment knob | Large knob used for rough focusing, especially at low power. |
| Fine adjustment knob | Small knob used for sharp focusing, especially at high power. |
| Arm and base | Provide support; the microscope is held by the arm while the base is supported. |
9.3.3 Light Microscope — Working
Light changes direction when it passes through glass. A convex lens bends light rays toward a point. In a compound microscope, the objective lens has a short focal length and first creates a real, inverted image of the specimen placed just outside its focal point. This image becomes the input for the eyepiece, which further enlarges it and produces the final highly magnified virtual image.
9.4 Microscopy Skills
9.4.1 Slide Preparation and Focusing
The chapter connects microscope observation with temporary mounts, including leaf peels of monocot and dicot leaves. The comparison includes epidermal cell shape, epidermal pattern, guard-cell shape and stomatal distribution.
9.4.2 Permanent Slides
Temporary mounts are useful only for a short time because the water drop dries, cells shrink and die, and the preparation may develop air bubbles, dried stain crystals and distorted cell shapes.
Permanent mount: The specimen is fixed, stained, dehydrated and
sealed under a coverslip in a mounting medium such as Canada balsam or DPX. This
allows the slide to be stored and used repeatedly for years.
| Feature | Temporary mount | Permanent mount |
|---|---|---|
| Purpose | Short-term observation | Long-term storage and repeated observation |
| Water / drying | Water may slowly dry out | Sealed mounting medium prevents drying |
| Specimen condition | Living/fresh cells may be observed | Specimen is fixed and preserved |
| Preparation | Relatively simple | Fixation → staining → dehydration → mounting/sealing |
| Examples of mounting medium | Water-based temporary preparation | Canada balsam or DPX |
9.4.3 Magnification
Magnification tells how many times larger the image appears compared with the actual object.
\(M = m_o \times m_e\)
where \(m_o\) is the magnification of the objective and \(m_e\) is the magnification of the eyepiece.
EXAMPLE10× eyepiece + 40×
objective
Total magnification = \(10 \times 40 = 400×\).
Total magnification = \(10 \times 40 = 400×\).
IMAGE SIZEActual size = image size ÷ total
magnification.
For example, an image measuring 5 mm at 100× corresponds to an actual size of \(5/100 = 0.05\) mm = 50 µm.
For example, an image measuring 5 mm at 100× corresponds to an actual size of \(5/100 = 0.05\) mm = 50 µm.
Calculation Check
- A 15× eyepiece and 10× objective are used. Find total magnification.
- If 4 cells fit across a 0.8 mm field of view, estimate the size of one cell.
- If an image is 5 mm at 100×, find the actual size.
9.5 Types of Microscopes
All microscopes perform the basic function of magnifying small objects, but they differ in how they form images and in their magnification and resolution ranges. The specimen and purpose of observation determine which microscope is appropriate.
9.5.1 Magnification vs Resolution — Big vs Sharp
Magnification: The factor by which a microscope enlarges the
image relative to the actual specimen size.
Resolution: The smallest distance between two close points that
can still be distinguished as separate. Higher resolution reveals finer detail.
Resolving power is the capacity to identify two closely placed
points as separate. The chapter states that it is expressed as the reciprocal of resolution; smaller
resolution means higher resolving power.
| Instrument | Resolution |
|---|---|
| Human eye | ~ \(1\times10^{-4}\) m ≈ 0.1 mm |
| Light microscope | ~ \(2\times10^{-7}\) m ≈ 0.2 µm |
| Electron microscope | ~ \(2\times10^{-10}\) m ≈ 0.2 nm |
9.5.2 Light (Compound) Microscope
- Uses visible light and glass lenses.
- Magnification is usually up to about 1000× in the chapter's description.
- Resolution is about 0.2 µm.
- Can be used to observe living cells such as moving protozoa or cheek cells.
9.5.3 Electron Microscopes
Electron microscopes use electron beams with extremely short wavelengths. The chapter gives about 0.005 nm for the electron wavelength compared with about 550 nm for visible light. Electrons are accelerated in a vacuum and focused using magnetic/electromagnetic lenses.
Why higher resolution? The shorter wavelength of the electrons
allows much finer spatial detail to be resolved than with visible light.
9.5.3.1 Transmission Electron Microscope (TEM)
- Uses an electron beam rather than light.
- Uses extremely thin specimen sections, about 50–90 nm thick, prepared using an ultramicrotome.
- Electrons pass through the ultra-thin specimen.
- Heavy-metal stains such as uranyl acetate and lead citrate provide contrast.
- Shows internal details such as mitochondria, ribosomes and viruses.
- Produces a two-dimensional image.
- Resolution can be around 0.1 nm.
9.5.3.2 Scanning Electron Microscope (SEM)
- Uses an electron beam rather than light.
- The beam scans the specimen surface.
- Electrons are reflected from the specimen.
- Conductive/heavy-metal coating is used.
- Produces 3D-like images of surfaces such as pollen grains, insect legs and microchips.
- Resolution can be around 1–20 nm.
9.5.3.3 Light Microscope vs TEM vs SEM
| Feature | Light microscope | TEM | SEM |
|---|---|---|---|
| Illumination | Visible light | Electron beam | Focused, scanned electron beam |
| Lenses | Glass convex/achromatic lenses | Electromagnetic coils | Electromagnetic coils |
| Section | Whole mounts / tissue sections; chapter gives up to several mm and 5–10 µm tissue sections | Ultra-thin, typically 50–90 nm | Surface only; no sectioning required for the described samples |
| Staining / coating | Basic dyes such as methylene blue and eosin | Dense metal compounds such as uranyl acetate and lead citrate | Conductive coating such as gold/palladium |
| Living cells | Yes | No | No |
| Resolution | ~0.2 µm | ~0.1 nm or better | ~1–10 nm in the comparison table |
| Magnification | Up to 1,500× in the comparison table | Up to 50 million× | Up to 2 million× |
| Preparation time | Minutes | Hours–days | Hours |
| Cost | ~₹10,000–50,000 | Very high (~₹50 lakh+) | High (~₹20–50 lakh) |
| Vacuum | No | Yes | Yes |
9.6 What is New in Microscopy? What are the Limits?
9.6.1 New Developments
Digital microscopes can show real-time images directly on a screen, allowing live images to be shared with many students. Super-resolution microscopes can reveal details smaller than the normal limits of conventional light microscopy.
9.6.2 Limitations
- With a light microscope, structures smaller than about 0.2 µm cannot be resolved because of the diffraction limit of light.
- Electron microscopes are costly, require a vacuum and need careful sample preparation; most samples require chemical processing and metal stains/coatings.
9.7 Where Do We Use Microscopes?
Hospitals & pathologyDiagnosis using blood, sputum and
tissue biopsies, including identifying malaria parasites in blood.
Science laboratoriesStudy stomata, plant and animal
tissues, plant diseases and other microscopic structures.
IndustryQuality checking of metals, plastics and electronic
chips using light and electron microscopes.
Police & forensicsExamination of fibres, hair, glass
fragments and blood stains.
EnvironmentChecking water samples for algae, protozoa and
pollution indicators.
Check Your Understanding — Solutions Hidden
Attempt each question first. Click Show Solution to reveal the answer. The activity-based prompts from the supplied chapter are intentionally not included in this exercise system.
1. A microscope has a 10X eyepiece and a 40X objective. (a)
What is its total magnification? (b) At this setting, the field of view is 0.4 mm. If 4 cells fit
across, estimate the size of one cell.
(a) Total magnification =
\(10\times40=400X\).
(b) Approximate cell size = field of view ÷ number of cells = \(0.4/4=0.1\) mm = 100 µm.
(b) Approximate cell size = field of view ÷ number of cells = \(0.4/4=0.1\) mm = 100 µm.
2. (a) You want to watch live protozoa moving in pond water.
Which microscope (light, phase-contrast, TEM, SEM) is best and why? (b) Neha wants to study the 3D
surface of a pollen grain. Which microscope should she choose and why?
(a) A phase-contrast microscope is the best
choice from the listed options because it enhances contrast without requiring chemical stains
and is useful for viewing living cells in their natural state.
(b) An SEM is suitable because it scans surfaces and produces 3D-like images of surface features.
(b) An SEM is suitable because it scans surfaces and produces 3D-like images of surface features.
3. Riya sees a sharp onion cell image at 100X, but when she
switches to 400X, the image is big but very blurred. Name the concept causing this problem. Explain
the reason.
The key concept is resolution. Increasing magnification makes
an image larger, but it does not automatically reveal more detail. If the resolving power is
insufficient, the enlarged image remains blurred. This is why magnification and resolution are
distinct concepts.
4. Draw a ray diagram of a compound microscope.
The ray diagram should show the specimen near the focal
region of the objective, the objective forming a real and inverted intermediate
image, and the eyepiece further magnifying that image to produce the final
virtual image. Refer to the ray diagram in Section 9.3.3 above.
5. Design a simple poster “How to take care of a microscope?”
with three do’s and three don’ts.
One suitable answer: Do: carry the microscope by the arm
while supporting the base; keep lenses clean using appropriate lens-cleaning material; store the
microscope safely after use. Don't: drag the microscope by the eyepiece; touch
optical surfaces with fingers; force the focus or mechanical parts.
6. At 40X total magnification, the field diameter is 4 mm.
Predict the field diameter at 400X magnification, assuming it is inversely proportional to
magnification.
Field diameter × magnification is constant. Thus \(4\times40 = d\times400\).
Therefore \(d=160/400=\mathbf{0.4\ mm}\).
7. A student accidentally traps many air bubbles while placing
the cover slip. How will this affect observation? Suggest two ways to avoid bubbles next time.
Air bubbles can obstruct the specimen and make parts of the image difficult to
observe clearly. To reduce bubbles, lower the coverslip gently at an angle and avoid placing it
abruptly onto the liquid.
8. Compare TEM and SEM in terms of: type of image and best use
(internal vs surface).
TEM: produces a 2D image and is best for internal details
because electrons pass through an ultra-thin specimen section. SEM: produces a
3D-like surface image and is best for studying surface features because the electron beam scans
the surface.
9. Plan a brief investigation using a school light microscope
to compare the purity of three water samples (tap water, RO-purified water, and pond water). Outline
the main steps and predict your expected observations.
Prepare comparable microscope slides from equal volumes of the three samples,
mount them similarly, observe under the same magnification and illumination, and record visible
particles or microorganisms. A reasonable prediction is that pond water may show more visible
biological material, while RO-purified water may show fewer visible particles. Microscopy alone
does not establish chemical purity; it only compares what is visibly present under the chosen
conditions.
10. Can we rely on electron microscopes for studying living
cells? Explain the reason.
No. According to the chapter, electron microscopes require a
vacuum and careful sample preparation, and the comparison states that living
cells cannot be observed because the vacuum and preparation conditions kill the cells.
11. List two ways microscopes are used in hospitals and one way
they are used in industries that manufacture mobile phones.
Hospitals/pathology laboratories use microscopes for examining blood
samples and tissue biopsies for disease diagnosis. In industry,
microscopes can be used to inspect the quality of electronic chips.
12. Imagine you are Robert Hooke. Write a 5–6 line diary entry
about what you felt when you first saw “little boxes” (cells) in cork.
Sample response: “Today I examined a thin slice of cork through my microscope.
I saw countless tiny box-like spaces arranged together like a honeycomb. The sight was unlike
anything visible to the naked eye. These little spaces appeared so distinct that I decided to
call them ‘cells’. I wonder what hidden world exists inside living things. This instrument has
opened an extraordinary new field of observation.”
13. Ananya says, “If we add more and more lenses, we can see
anything, even atoms, with a school microscope.” Use the idea of resolution to correct this
statement.
Adding lenses can increase magnification, but useful detail
is limited by resolution. If two points cannot be resolved as separate,
increasing magnification only makes the unresolved image larger. A school light microscope
therefore cannot reveal arbitrarily small structures such as atoms simply by adding more lenses.
14. Nishant wants to observe the effect of concentrated salt
solution on cells of Rhoeo leaf and also wants to keep slides for future reference. (a) Which type
of mount should be used for this purpose? Give reason. (b) Will the same slide be suitable for
long-term storage? Explain.
For observing the effect of concentrated salt solution on living Rhoeo cells,
a temporary mount is appropriate because the cells need to be observed in a
fresh preparation. The same temporary slide is not suitable for long-term storage because water
can dry, cells can shrink and the preparation can deteriorate. A separately prepared
permanent mount is used for long-term preservation.
15. Why is it important to fix and dehydrate cheek cells before
mounting in Canada Balsam for school laboratory storage? Predict the consequences if a student
inadvertently skipped the fixation and dehydration steps before mounting the specimen in Canada
Balsam.
Fixation preserves the specimen's structure, while dehydration removes water
before sealing it in the mounting medium. Skipping these steps can leave water and unpreserved
material in the preparation, causing deterioration, poor preservation and reduced clarity over
time. The supplied chapter identifies fixation, staining, dehydration and sealing as the steps
used to prepare permanent mounts.
Worksheets
Practice sheets for microscope parts, magnification, resolution, microscope selection and TEM/SEM comparison will be added here.
Coming SoonQuick Revision
| Concept | Remember |
|---|---|
| Microscope | Instrument used to observe tiny organisms or structures by magnifying them. |
| Human-eye resolution | About 0.1 mm (100 µm) at the near point according to the chapter. |
| Robert Hooke | Observed cork in 1665 and coined the term “cell”. |
| Leeuwenhoek | Observed bacteria and protozoa using powerful simple lenses; called them “animalcules”. |
| Magnification | How many times larger the image is than the actual specimen. |
| Resolution | Smallest distance between two points that can still be distinguished separately. |
| Simple microscope | Single lens. |
| Compound microscope | Objective + eyepiece; the objective forms the first real, inverted image. |
| Fluorescence microscopy | Specialised dyes are excited by high-intensity light so structures glow. |
| Phase contrast | Enhances contrast without chemical staining; useful for living cells. |
| Total magnification | \(M=m_o\times m_e\). |
| Light microscope | Visible light + glass lenses; resolution about 0.2 µm. |
| Electron microscope | Electron beam + electromagnetic lenses; requires vacuum. |
| TEM | Electrons pass through ultra-thin specimen; internal details; 2D image. |
| SEM | Electron beam scans surface; surface details; 3D-like image. |
| Temporary mount | Short-term preparation; may dry and deteriorate. |
| Permanent mount | Fixed, stained, dehydrated and sealed for long-term storage. |
| Digital microscopy | Real-time image can be displayed on a screen. |
| Super-resolution | Can reveal details smaller than conventional light-microscope limits. |
| Applications | Hospitals, laboratories, industry, forensics and environmental examination. |
Core distinction: Magnification makes an image bigger; resolution
determines how much fine detail can actually be distinguished.
Test Yourself
A chapter test covering microscope parts, magnification, resolution, microscopy history, light microscopy, TEM, SEM and applications will be added here.
Coming Soon