Optical Microscope
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of cell structure and the need for magnification to observe cellular components (from GCSE). You can test your knowledge on these below.
Why are stains, such as iodine solution, used in microscopy?
To stain the cells and their organelles to enable you to see them more clearly.
Give two disadvantages of a light microscope.
- They have a lower magnification
- They have a lower resolution (than electron microscopes)
Why do specimens need to be thin when viewed under a light microscope?
To allow light to pass through, ensuring a clear image of each cell and their organelles.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains optical microscope or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of Microscope
We use microscopes to look at cells because:
- Cells are too small to be seen with the naked eye: Most cells range from 1-100µm in size, which is below the resolution limit of the human eye.
- To observe cell structures in detail: Organelles like the nucleus, mitochondria, and chloroplasts can only be seen clearly using high magnification and resolution.
- To study cell processes: Microscopes allow scientists to see cell division (mitosis & meiosis), movement (e.g., in amoeba), and interactions between cells.
- Medical and scientific research: Microscopes help diagnose diseases by examining blood cells, bacteria, and tissues.
The Optical (Light) Microscope
- Uses visible light and glass lenses to magnify images.
- Commonly used in schools, laboratories, and medical settings.
How to Prepare a Temporary Mount for a Light Microscope
- Cut a very thin sample – ideally you want a single layer of cells- allows light to pass through for clear image resolution.
- Add a drop of water to the slide then place the specimen on to the slide, making sure it is flat.
- Stain the sample (e.g., iodine solution for onion cells, methylene blue for staining DNA in the root tip squash practical when observing chromosomes) – enhances contrast to highlight different organelles.
- Slowly lower a cover slip at an angle to avoid air bubbles that could distort the image.
- Gently squash the coverslip to apply pressure and ensure the sample is thin / a single layer of cells.
Onion cells stained with iodine solution and seen under the optical microscope.
The cell walls and the nuclei are stained and visible here.
How to Place the Slide on the Stage and Focus the Microscope
Placing the Slide on the Stage
- Carefully pick up the prepared slide by holding its edges to avoid fingerprints or damage.
- Place the slide flat on the stage, ensuring the specimen is positioned over the hole where light passes through.
- Secure the slide using the stage clips to prevent movement.
Focusing the Microscope
- Start with the lowest power objective lens (usually ×4 or ×10) to get a wide field of view.
- Look through the eyepiece and use the coarse focus knob to move the stage up or down until the image comes into focus.
- Switch to a higher magnification (e.g., ×40) if needed, and use the fine focus knob for precise focusing.
- Adjust the light intensity using the diaphragm if the image is too bright or too dark.
Tip
Always start with low power and bring the stage close to the lens before slowly adjusting focus and moving the stage down to avoid damaging the slide.
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Advantages and Disadvantages of Optical Microscopes
Advantages of Optical Microscopes
- Can observe living cells.
- Simple sample preparation.
- Users observe natural colours of specimens (unless stained).
- Relatively inexpensive compared to electron microscopes.
Disadvantages of Optical Microscopes
- Lower magnification (up to ×1500).
- Lower resolution (limited to 0.2µm due to wavelength of light).
- Light has a longer wavelength than electrons, hence the lower resolution of the light microscope.
- Can’t see smaller organelles, such as ribosomes / lysosomes.
Key Terms
- Magnification: How many times larger the image is compared to the actual object.
- Formula: Magnification = Image Size / Actual Size
- Resolution: The ability to distinguish between two separate points.
- The higher the resolution, the closer two objects can be where they can still be distinguished as separate.
- Higher resolution increases the clarity of the image and allows you to see more detail of organelles.
- The resolution is given as a distance. This is the distance between two points that the microscope can distinguish between.
- Staining: Using dyes to increase contrast between different parts of the cell and highlight organelles.
- Cover Slip: A thin glass placed over the specimen to protect it and make the specimen flat and thin.
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Exam Tip
You do not have to learn the magnification and resolution of different types of microscope. Just make sure you can compare them using terms such as higher and lower.
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A student prepares a temporary mount of an onion cell to view under a light microscope. Explain why the specimen must be thin and why a stain is used. (4 marks)
- The specimen must be thin to allow light to pass through, ensuring clear visibility of cells / organelles.
- A stain is used to provide contrast, allowing organelles like the nucleus to be seen more clearly.
- The stain binds to specific structures to differentiate cell components.
- Without staining, cells and organelles may be transparent, making it difficult to observe details.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!