(2) Microscopy

The design, function and use of an optical light microscope and slide preparation

Although scientists use the more powerful electron microscopes in research projects, many day-to-day investigations are performed using light microscopes.

Examples include forensic science laboratories (e.g. evidence samples), hospital laboratories (e.g. examining the state/count of blood cells or looking for cancer cells by a pathologist).

 

The basic design of an optical visible light microscope

The basic design of an optical light microscope is shown in the left diagram. The back arch acts as a handle to carry the microscope around.

1. The eyepiece, through which you look at the image, is at the top of the tube that can move up and down when focussing the microscope. The eyepiece lens also magnifies the image produced by the objective lens.

2. There are two knobs for focussing - the course focussing knob is turned to get the image 'roughly' in focus and 'perfect' focus is enabled by turning the fine adjustment knob - moves in smaller increments on turning. The focussing knobs either move the stage or the eyepiece tube up or down to bring the image into focus.

3. On the stage, the prepared microscope slide (see below) is supported and held in place on the stage by clips - the slide needs to static to get good focus!

4. The lower end of the tube is connected to (usually) several objective lenses with different magnifications e.g. they might be marked X5, X10 or X20 etc. When at least two lenses are used it is referred to as a compound microscope.

5. The iris diaphragm under the stage controls the light intensity.

6. A mirror under the stage and iris directs a beam of light upwards through the stage, iris, slide and lens and up the tube to the eyepiece where you observe the image. You need a good bright light source to see any magnified image clearly.

 

Microscope slide preparation

The biological specimen you want to examine must be transparent, so you need a thin slice of it to let the light through.

To examine a specimen like plant or animal cells under a microscope you need to prepare microscope slide.

Live cells can be mounted in a drop of water or dilute sodium chloride salt solution (NaCl_(aq), saline).

Since most cells appear colourless you can apply a stain (dye) to give colour and enhance contrast.

You might use methylene blue to stain animal cells or iodine to stain plant cells (iodine turns starch dark blue).

A microscope slide is a rectangular thin strip of glass or clear hard plastic onto which you mount the specimen.

You need a very thin slice of the material under examination, if it is too thick, the image is too complicated and likely to be blurred - so you may have thin out a thick specimen.

The specimen to be examined must be thin and transparent to visible light.

The most common specimen for first time use of a microscope are onion cells.

With a pipette you place a drop of pure water (or a special clear liquid called a mountant) onto the middle of a clean slide which helps keep the specimen in place.

Cut up an onion to separate it into layers and with tweezers peel off a thin strip of epidermal tissue and place it in the water on the slide.

Add a drop of dilute iodine solution to the water and onion cells.

The iodine solutions acts as a stain to highlight particular features of the specimen - the thin layers of onion cells.

If all of the sample is colourless you might not see much detail.

Different stains can be used to highlight different structures e.g.

For microscopic examination of cell structure: eosin dye is used to stain cytoplasm, methylene blue dye stains DNA and iodine to stain starch.

Then place a cover slip over the specimen. A cover slip is a very thin square of glass or hard transparent plastic.

The cover slip should be placed on with great care and lowered into position so that no air bubbles are trapped underneath.

You can use a mounted needle to hold the cover slip up at an angle and slowly and carefully lower it onto the slide.

Press down gently making sure no air bubbles are trapped under the cover slip.

Air bubbles will either obscure or distort the image you are attempting to observe.

The microscope slide is now ready to be examined with the light microscope.

 

Using the microscope - how to use a microscope prior to section (3)

Carefully clip the prepared slide onto the stage and make sure the specimen and cover slide are directly underneath the objective lens.

Initially use the less powerful objective lens to focus on the specimen, but first use the course adjustment knob to raise the stage to just below the objective lens.

It might be difficult to locate individual cells if you start off with too high a power lens, but a low power lens gives you an overall picture of the layout of the cells e.g. in human or plant tissue. It can also be easier to a cell count.

The circular area you see through the lens of a microscope is called the field of view.

With a very high powered lens the field of view is very small, almost at the level of few individual cells.

With a low power lens the field of view is much wider e.g. multiple cells of a thin slice of tissue.

Once you are focussed and made the appropriate observations and sketches, you can then increase the lens power to look for more detail.

Adjust the mirror and light source so the image seems bright, though not necessarily in focus yet.

Looking down the eyepiece, use the course adjustment knob to move the stage downwards until the image is roughly in focus.

Then use the fine adjustment knob to fully focus on the object and get a clear sharp image.

You then have the option of swapping to a higher powered objective lens to produce a more magnified image.

For scale measurements you can position a transparent ruler to measure the diameter of your field of view (FOV).

If you change to a more powerful objective lens for greater magnification you will reduce the FOV.

e.g. if you FOV was 10 mm and you swap to a lens that is five times more powerful your FOV is reduced from 10 mm to 10 ÷ 5 = 2 mm.

---

Key points - Summary of ideas

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Revision notes on using an optical (light) microscope, including its design, function, how to use it, and the slide preparation process.

---

Optical Light Microscope – GCSE Level Revision Notes

1. Design and Main Parts of a Light Microscope

2. Function of a Light Microscope

Light microscopes use visible light and a combination of glass lenses to magnify biological specimens. They allow us to:

• Observe living or dead cells and tissues.

• Study cell structure (e.g. nucleus, cell wall, cytoplasm).

• Investigate basic cell processes like mitosis or osmosis.

---

3. How to Use a Light Microscope

1. Place the slide on the stage and secure it using stage clips.

2. Select the lowest magnification objective lens (e.g. 4× or 10×).

3. Use the coarse focus knob to bring the specimen roughly into view.

4. Adjust the diaphragm for the best lighting.

5. Use the fine focus knob to sharpen the image.

6. To observe in greater detail, switch to a higher magnification lens, and adjust the fine focus again.

Total magnification = Eyepiece × Objective lens

---

4. Slide Preparation for Biological Samples

a) Dry Mount (for non-living specimens)

• Place the specimen (e.g. a leaf) on the slide.

• Cover with a cover slip.

b) Wet Mount (for living or delicate specimens)

1. Place a drop of water or stain (e.g. iodine or methylene blue) on the slide.

2. Add the specimen (e.g. cheek cell, onion epidermis).

3. Lower the cover slip at an angle using a mounted needle to avoid air bubbles.

4. Remove excess liquid with a tissue if necessary.

Stains help make organelles more visible by increasing contrast.

---

5. Why It’s Important for Understanding Human Biology

• Helps identify and understand human cell types (e.g. red and white blood cells, epithelial cells).

• Used in medical diagnostics to detect abnormalities (e.g. sickle cell anaemia, infections).

• Enables students to visualize cell structure and relate it to function.

• Supports learning of key concepts like Cell Theory, mitosis, and tissue organization.

---

Top Tips for Exams

• Be able to label the microscope and describe its functions.

• Practise calculating magnification and interpreting microscope images.

• Understand the steps of slide preparation and why staining is used.

---

Keywords, phrases and learning objectives for this part on the structure, function and use of a microscope including slide preparation for investigating the structure of biological materials

Know the design and function of the parts of an optical microscope.

Know how to use a microscope, making up a side ready for study,

Be able to describe how to use an optical light microscope and how to prepare slides for the examination and investigation of biological materials.

Understand why a staining technique is usually required to see the structure of cells clearly.

---

WHAT NEXT?

TOP OF PAGE

INDEX of biology notes on microscope investigations

INDEX of all my BIOLOGY NOTES

This is a BIG website, so try using the [SEARCH BOX], it maybe quicker than the many indexes!

email doc brown - comments - query?

Basic Science Quizzes for UK KS3 science students aged ~12-14, ~US grades 6-8

Biology * Chemistry * Physics for UK GCSE level students aged ~14-16, ~US grades 9-10

Advanced Level Chemistry for pre-university age ~16-18 ~US grades 11-12, K12 Honors

Find your GCSE/IGCSE science course for more help links to all science revision notes

---

how to use an optical light microscope for syllabus-specifications for students taking the IGCSE/GCSE level biology examinations, summary revision notes key points on how to use an optical light microscope for students studying AQA igcse/gcse biology notes on how to use an optical light microscope, Edexcel gcse biology notes on how to use an optical light microscope,  OCR 21st century GCSE biology notes on how to use an optical light microscope, OCR gateway GCSE biology notes on how to use an optical light microscope, WJEC gcse biology notes on how to use an optical light microscope, CCEA gcse biology notes on how to use an optical light microscope, CIE Cambridge igcse biology, notes on how to use an optical light microscope useful for US grade 9-10 biology student courses, Explaining importance of how to prepare a slide for microscope investigation in GCSE level biology, What you need to know about how to prepare a slide for microscope investigation for GCSE level biology, Explaining use of how to prepare a slide for microscope investigation knowledge in GCSE level biology, Examples of how to prepare a slide for microscope investigation explained when studying GCSE level biology, What is the significance of how to prepare a slide for microscope investigation in GCSE level biology, describing explaining theory of how to prepare a slide for microscope investigation when studying GCSE level biology, exam revision notes for how to prepare a slide for microscope investigation, online help for understanding how to prepare a slide for microscope investigation in GCSE biology, what do I need to learn about how to prepare a slide for microscope investigation? what do I need to know about how to prepare a slide for microscope investigation for GCSE biology exams, how to prepare for questions on how to prepare a slide for microscope investigation in GCSE biology examination?

---