An overview of a microscope
A microscope is an instrument used to see objects that are too small to be seen by the naked eye. Microscopes are commonly used in science laboratories and classrooms to visualize all kinds of tiny objects, such as cells, microorganisms, tissue structures, materials, and electronics.
The function of the microscopes is to provide magnification (enlarging the image) and give the images a contrast (making them stand out of the background). To do so, microscopes are made up of a few lenses. Each lens with its own magnification powers and focusing strength.
[In this image] The name “microscope” came from two words – “micro” and “scope”.
“Micro” means small or tiny. “Scope” means to view or to observe. Therefore, a microscope can be understood as an instrument to see tiny things.
Image credit: Rs Science
Magnification and instruments
A microscope is an optical instrument to view small objects by enlarging them with convex lenses. Depending on the design, a light microscope usually has a magnification ranging from 10x to 1000x. Higher magnification requires the usage of electron microscopes.
Below is a summary of how much magnification is suitable for what type of object.
|Hair (approx. 0.1 mm)
|2x – 5x
|Plant and insect in detail
|10x – 20x
|Insect’s compound eyes
|Daphnia, Rotifers, Water bears
|Cheek cells, Onion skin cells
|800x – 1,500x
|Red blood cells (8µm), bacteria (1µm)
|2,000x – 1,000,000x
|Objects smaller than 1μm, such as a virus (100 nm) and DNA (2 nm)
An overview of the biological scale
To understand the magnitude of the biological scale, the best way is to find a good representative example in that range as a reference object. Let’s see what we have here.
An atom is the smallest unit of ordinary matter. Atoms are around 100 picometers (10−10 m or 0.1 nm) across. Usually, we use 1 angstrom (1Å = 10−10 m) for the size of an atom.
Small biological molecules
Biological molecules refer to all the molecules in organisms that are essential to the biological functions of cells. Small biological molecules could be chemicals like oxygen and water as well as units to build macromolecules (DNA, complex carbohydrates, proteins, and lipids). These molecules range from 0.2 nm – 1 nm (10−9 m) .
DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix. The DNA double helix has a diameter of 2 nm. The space between two base pairs is 0.34 nm. The distance between each helix turn is 3.4 nm (including ~10 base pairs).
DNA molecules are too thin to be seen by a light microscope. However, when the cells are dividing, DNA threads condense into chromosomes, which can be seen by Methylene blue staining under the light microscope.
[In this image] Condensed chromosomes in plant root cells under a 1000x magnification.
A virus is a tiny structure (virion) that consists of a single molecule of nucleic acids (RNA or DNA) surrounded by a protein coat, and/or a lipid envelope. Viruses have some of the properties of life. However, they do not have a cellular structure, nor can they reproduce without a host cell. For these reasons, viruses are not generally considered to be alive.
The size of viruses ranges from 20 to 400 nm, which is too small to be seen with an optical microscope. It would be best if you had a transmission electron microscope to see viruses.
[In this image] Virus structure and size.
Bacteria are prokaryotic cells — they don’t have nuclei and other organelles. Bacteria have diverse shapes and sizes. For spherical bacteria (Cocci sp.), the diameter can range from 0.2-2 µm. The length can range from 1-10 µm for filamentous or rod-shaped bacteria (Bacilli sp.).
The most well-known bacteria: E. coli, their average size is ~1.5 µm in diameter and 2-6 µm in length.
[In this figure] The bacteria can be grouped into three basic shapes, coccus (spherical, plural: cocci), bacillus (rod-shaped, plural: bacilli), and spiral.
Baker’s yeast (Saccharomyces cerevisiae) is an example of small eukaryotic cells. They are about 4 µm in diameter and can reproduce by budding a new cell from the parental cell.
Yeasts are one of the everyday things you should take a look at under a microscope.
[In this figure] Microscopic view of yeast cells.
Yeast cells are oval in shape and some of them are dividing (budding yeasts).
Image credit: Rs Science
Animal cells and Plant cells
Both animals and plants are multicellular organisms. The cells are the “building blocks” of animal and plant bodies. Animal cells typically range from 8 µm to 50 µm. However, their sizes and shapes could be very different based on their functions. For example, our neuron can extend its protrusion, called the axon, to send signals to a muscle fiber that is far away from the neuron. These neurons range from a tiny fraction of an inch (or centimeter) to three feet (about one meter) or more in length.
Plant cells are usually bigger than animal cells and are range from 20 – 100 µm. Plant cells have a cell wall to maintain a fixed cell shape.
[In this figure] Different cell sizes and shapes comparison.
Human cells are relatively small. Some cells (like neurons and muscle cells) can be long. Fat cells (adipocytes) could be big because most of their cytoplasm is occupied by a huge oil droplet. On the other hand, plant cells and single-celled organisms (like paramecium or amoeba) are larger in size and require cytoplasmic streaming to distribute substances in these cells.
Paramecium (pair-ah-me-see-um; plural, Paramecia) is a unicellular (single-celled) living organism with a shape resembling a slipper. Paramecium is naturally found in aquatic habitats. They range 50 to 300 µm in length.
[In this figure] Sizes of paramecia vary from species to species.
P. caudatum is among the largest protozoan and can grow up to 200-300 µm. It is barely visible to the naked eye. Another common paramecium, called P. aurelia, is smaller (50-150 µm). P. caudatum is more elongated and P. aurelia is more ovoid in shape.
Photo credit: P. caudatum (Deuterostome), P. aurelia (Barfooz on Wiki)
You may notice that the cell sizes of single-celled microorganisms could be much bigger than cells in multicellular animals and plants. Amoebas and Stentors are two more examples of large single-celled microorganisms.
Hairstyling is a part of our everyday life, but have you ever really looked at your hair under a microscope? The human hair is around 60-100 micrometers in width.
How a microscope works
For a light microscope, visible light passes through the specimen and then a series of lenses. Lenses bend the light, which results in magnification.
[In this figure] Principles of light microscopy.
(Modified from Atlas RM: Principles of microbiology, St Louis, 2006, Mosby.)
Photo credit: Role of Microscopy
To better understand how the magnification is generated, let’s start with only one convex lens. This kind of instrument is called a simple microscope or magnifier.
The image below shows how to draw a ray diagram for an object nearer than the lens F. “F” is the focal point of this convex lens.
The bottom of the object is placed on the principal axis. Two rays of light are drawn from the top of the object. The first ray of light is parallel to the principal axis. Any light ray parallel to the principal axis will be refracted, change direction, and cross the principal axis at the focal point.
The second ray of light goes from the top of the object and passes straight through the center of the lens. This ray does not refract.
As you can see, the rays are diverging (moving apart) on the right side of the lens. The eye looks back along with the rays that seem to have come from a point behind the object where the two rays of the light cross. This is where you draw the top of the virtual image. The bottom of the image is still on the principal axis.
The image made by a magnifying glass is virtual, upright, and bigger than the object. The image is called virtual because the light rays never really go there. The virtual light rays are drawn as dotted lines.
You can experiment with the DIY magnifying glasses with a recycled plastic bottle or water-filled balloon.
Now we move forward to a compound microscope of two convex lenses (called Objective and Ocular lenses, respectively).
As you can see in the above image, the actual object (1) will be placed near but outside the focus point of the Objective lens. This generates an inverted and bigger image (2) between two lenses. This image is formed by real light rays and setting within the focus range of the Ocular lens. Now, if you observe this image behind the Ocular lens, the image will be magnified again. The final image (3) is virtual and inverted with the combined magnification contributed by both Objective and Ocular lenses.
The total magnification of a compound microscope is determined by the combination of Objective and Ocular Lenses. Assuming you have 10x eyepieces (ocular lenses) and 100x objective, the total magnification of this combination is 1,000x (10×100 = 1000).
[In the figure] The low to high magnifications of vicia root tip and human blood cells.
Image source: Rs Science