Where does the light come from in a compound microscope?

Everyone is familiar with a compound microscope to some degree, since it’s what we often see in school and on television. Many of us even had the experience of using one at some point throughout our education.

Compound light microscopes are small, simple, and convenient. They are also inexpensive, which is partly why they are so popular and commonly seen just about everywhere. In fact, if you are passionate about studying microorganisms, you can even purchase one for your home laboratory.

Generally speaking, these are excellent and versatile microscopes for studying the microscopic world. If you are interested in one, here is everything you need to know about a compound light microscope:

This in-depth guide contains:

What is a compound light microscope?

A compound light microscope is a type of light microscope that uses a compound lens system, meaning, it operates through two sets of lenses to magnify the image of a specimen. It’s an upright microscope that produces a two-dimensional image and has a higher magnification than a stereoscopic microscope.

It also goes by a couple of other names, the most common being simply a compound microscope, since it has at least two lenses. It’s also referred to as a high power microscope due to its high magnification, as well as a biological microscope, as it’s most often used to view cells of living organisms.

Origin and history

The first ever microscope was a single microscope, which essentially featured a single lens and a sample holder. It eventually evolved into a rudimentary version of the compound microscope, which was made of collapsing tubes that have a combined 9x magnification. This was invented by Zacharias Jansen way back in 1595.

Since then, compound light microscopes have developed into more complex devices with a much higher level of magnification and resolution, making them suitable for studying micro specimens.

What is a compound light microscope used for?

A compound microscope is a great imaging tool for viewing microscopic specimens that are otherwise not visible to the naked eye, mainly because of its high magnification, which can reach up to 1000x or more.

Extremely valuable in the fields of microbiology and bacteriology, a compound microscope can be used to study living cells such as blood cells, wherein the microscope enables you to study its cell structure and more. This is actually why compound microscopes are often referred to as biological microscopes since they are primarily used to examine living specimens.

What you can see

Somewhat larger and more visible specimens such as live protists and metazoans, plant cells including algae, and even pond water, can be examined under a compound microscope with just a simple unstained wet mount.

Whereas, smaller samples including blood, chromosomes, bacteria, organelles, thick tissue sections, and other protists and metazoans, need to be prepared first by staining the sample in order for the details to be clearly visible.

Types of a compound microscope

There are many different types of compound light microscopes, but most people have the impression that these are only limited to biological microscopes, since these are the most commonly used in a variety of practices, especially in school. Here are some types of compound microscopes:

Biological microscope

A biological microscope is referred to as such since it’s used mainly for studying living organisms and cells. These microscopes are also often generalized as bright field microscopes or transmitted light microscopes, because those are the microscopy techniques involved in how the microscope functions.

Fluorescence microscope

A fluorescence microscope, also called a confocal microscope, is a kind of biological microscope that operates by using different light colors and wavelengths over-dyed specimen samples in order for the dye to interact with the light, after which the resulting image is scanned. Multiple scans of the specimen can be combined in order to produce a 3D image.

Phase contrast microscope

A phase contrast microscope is a somewhat specialized compound microscope that utilizes a certain kind of objective lens (a special phase contrast) and a phase condenser or slider. This is arguably easier to use since it can bring out the contrast of a specimen image without the use of any staining technique, and is great for viewing blood cells and bacteria.

Polarizing microscope

A polarizing microscope is a combination of two imaging techniques, namely, bright field microscopy and polarization. It utilizes an analyzer and a polarizer to cross-polarize the light and showcase more details of the specimen. It’s most often used in chemistry, pharmacy, geology, and petrology.

Metallurgical microscope

A metallurgical microscope is also another type of compound microscope. It produces images through using reflected light, or the combination of reflected and transmitted light. Contrary to common compound microscopes, the light comes from above and passes through the objective lens. Alternatively, this microscope may make use of dark field microscopy techniques.

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What are the parts of a compound light microscope?

Going by its name, compound microscopes should have at least two lenses- the objective lens, compounded by the ocular lens. But the reality is far more complex than that. Depending on how sophisticated the microscope is, the more (and better) parts it will have. But, as with all light microscopes, the most basic parts of a compound microscope are the lenses and the eyepiece.

Objective lenses

A compound microscope will normally have around three to five objective lenses, each with a magnification of 4x to 100x. These lenses are located on the rotating nosepiece, and are the most crucial in magnifying the specimen in order to see it bigger, better, and in more detail.

There are different classifications and specifications of objective lenses, but what’s important to remember is that these are the ones that gather light from the specimen, which is what produces the specimen’s real magnified image.

Ocular lenses

The eyepiece of the microscope is where the magnified image of the specimen can be viewed and analyzed. This eyepiece is, in fact, another type of lens, which is an ocular lens, and which also has a magnification, which is typically around 10x. A compound microscope’s eyepiece is usually binocular, but it can sometimes be monocular or trinocular.

Monocular eyepieces are basic and lightweight but often difficult to use, whereas binocular eyepieces are much more comfortable and are the most common option, while trinocular eyepieces are meant for two people studying the specimen simultaneously.

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Light source

The microscope’s light source is one of the three most important parts of any light microscope, since this is what illuminates the specimen and lets you be able to see it clearly. This light source is typically located at the bottom of the microscope, below the sample holder. Many modern microscopes are now equipped with bright LED bulbs as the main light source.

Other parts

A compound microscope is also composed of several other parts, including the following:

• The condenser focuses the light from the light source or illuminator directly onto the sample above. It’s attached to the base of the microscope, where the switch is typically located, if applicable.
• The stage or specimen holder is where slides of specimen samples are placed. It features stage clips, which are designed to securely hold the slides in place. It also has an opening at the center, called the aperture, as well as an iris diaphragm.
• The body of the microscope features several control dials, including the fine focus, coarse focus, diopter adjustment, stage control, and rack stop.
• The upper half of the microscope is composed of the arm designed to carry the microscope, the revolving nosepiece that holds the objective lenses, and the eyepiece at the end of the body tube.

How does a compound light microscope work?

We have mentioned earlier that compound microscopes are a type of light microscope- specifically, bright field microscopes. This means that they use a light source to illuminate, magnify, and view the specimen.

A compound light microscope has its own light source in its base. The incandescent light from the light source is reflected by a condenser lens beneath the specimen, and the light passes through the specimen, up to the objective lens, then the projector lens sends the magnified image onto the eyepiece.

Bright field microscopy

Bright field microscopy is an imaging technique where the specimen is lit from below and viewed from above, and wherein the sample’s image contrast depends on its absorption of light. This is the most elementary form of microscope illumination technique that is used by the first microscopes as well as many modern microscopes.

A brighter and more focused or concentrated light shows more detail, especially on thin and high contrast specimens, and works better for higher magnification. Conversely, fully opaque and transparent specimens are low contrast, so these need to be stained beforehand, or even viewed through other imaging techniques.

What factors affect the microscope’s imaging?

How and how well a compound microscope can produce a magnified image of a specimen really just depends on how well it performs its core purpose, which is to magnify an image, and to do it in such a way that the details are clearly visible and discernible.

This means that what you’ll see through the microscope depends on how far it “zooms” into the specimen, how well it preserves the details of that image, and even how close the specimen is to the light source and lenses to begin with.

Magnification

To magnify an image basically means to view it larger. A compound microscope can have a magnification of anywhere from 40x to 1000x, depending on the individual magnification of each objective and ocular lens.

The thing is,what usually happens with most microscopes is that the higher the magnification, the lower the image resolution. This is especially since a certain degree of distortion happens when viewing smaller specimens in the first place. A common solution is to use immersion oil on the lens for microscopes with at least 100x magnification, in order to concentrate the light better.

How to calculate:

The total magnification of the microscope can be calculated by multiplying the magnification of the objective lenses, which ranges from 4x to 100x for each of the microscope’s 3 to 5 lenses, with the magnification of the ocular lens or eyepiece, which is usually around 10x.

So, for example you have a 10x lens and a 10x eyepiece, the total magnification is 100x.

Resolution

Since it’s equally important to have a clearly detailed image, the microscope’s resolution or resolving power also comes into play. This is what increases the amount of visible detail and presents tiny objects that are close together into separate discernable images.

What primarily affects this, aside from the microscope’s optical quality, is the distance of the light wavelength being used. To put it simply, shorter wavelengths produce higher resolution images and vice versa.

What you can do:

While it’s certainly possible to adjust the light source, including its intensity, concentration, wavelength, and so on, the best way to ensure a high resolution image is to make sure that the specimen samples are well prepared to begin with.

Of course, you can also employ other imaging techniques alongside bright field microscopy, such as polarization, which is used to identify details that may not be visible under white light.

Staining the specimen

Natural pigmentation is important to ensure contrast, but you can make use of stains and staining techniques to further enhance the contrast and improve your viewing experience. This is especially important when dealing with fully transparent and opaque specimens. This includes using gram stain, methylene blue, and fuchsin for bacteria, cell nuclei, and smooth muscle cells respectively.

Working distance

Working distance refers to how close the nosepiece is to the specimen slide. The smaller and thinner the specimen, the closer you want to bridge this gap, in order for the light to be as bright and focused as possible, resulting in a higher magnification with better resolution.

The problem is, it’s difficult to close that distance, especially if the microscope has oil immersion lenses. There’s always the possibility of the nosepiece coming into contact with the slide, therefore damaging the specimen. This is where mechanical stages come in for microscopes with a 400x magnification or higher, since these allow for a safer and more precise adjustment of the slide at fractional distances.

How much does a compound light microscope cost?

The answer to this really depends on the kind of microscope, a.k.a whether it’s a basic or sophisticated model. Entry-level microscopes can cost around $200 to $1000, sometimes even cheaper, and are great for basic school work and science classes, or for older kids and hobbyists.

On the other hand, mid-range to high-end compound microscopes can be priced at tens of thousands of dollars, since these are mainly used in advanced research facilities. These are generally more stable, versatile, and resilient.

Along with the hefty price tag comes an assortment of specialized features, such as tension-free and apochromatic objectives, condensers with different contrast methods including dark field microscopy, a bunch of quality spare parts, and so on.

A microscope is a delicate scientific device that requires utmost care and delicacy when using and taking care of, so it does not get damaged or broken. A good quality microscope should last an entire lifetime without any issues, as long as it is used properly.

When using the microscope for the first time, it’s important to first familiarize yourself with each of its parts, how it’s used, and how the microscope is supposed to work.

It’s also important to remember that specimens need to be prepared and placed on a slide, which should be covered by another slide to protect it from coming into contact with the lenses or any part of the microscope.

Here are the steps on how to use a compound microscope:

Part 1: assembly

1. Carry the microscope safely by lifting it through the arm and using your other hand to support the base. Place it on a flat and even table with the arm facing towards you.
2. For the initial setup of the microscope, raise the body tube by turning the coarse focus or coarse adjustment knob. Then, revolve the nosepiece or turret so you can start with the low power objective lens. Make sure it clicks into place.
3. Look through the eyepiece and adjust the iris diaphragm and mirror until a circle of bright white light is clearly visible. Now, you’re ready to begin.

Part 2: use

1. Carefully place the slide on the stage, with the specimen at the center of the aperture or stage opening. Do this while looking through the eyepiece, and secure the slide with the stage clips once the entire specimen is centered.
2. Looking at the side of the microscope, carefully lower the body tube by adjusting the coarse focus, until the objective lens is almost touching the slide. Then, look through the eyepiece again and slowly adjust the knob further in order to focus the image.
3. If you want to shift to the high power objective lens, simply revolve the nosepiece slowly and carefully until the lens clicks into place, making sure that it doesn’t touch the specimen. Afterwards, adjust the knob again to focus the image.

Part 3: aftercare

1. Once you are finished viewing the specimen, adjust the coarse focus again, this time to lower the stage. Then, reposition the low power objective lens and click it into place before finally removing the slide.
2. Switch off the microscope, and clean if necessary. Only use a special lens paper to wipe off the objective lenses and the eyepiece, and avoid touching these with your bare hands. Even toilet paper and paper towels can scratch these sensitive glasses.
3. Finally, carefully and properly store the microscope in its designated storage place. Make sure to cover it with a sheet or place it in its box to protect it from dust and dirt.

Things to Remember

A compound microscope is a type of light microscope that uses a compound lens system to magnify specimens for up to 1000x or more. It’s made up of at least one objective lens and at least one ocular lens, as well as a light source, condenser, and other essential parts.

It’s used for viewing and studying minute and specimen microscopy that is not visible to the naked eye, and is extremely valuable in a variety of fields. Magnification, resolution, and working distance must all play together in order to produce a clear and detailed magnified image of the specimen being viewed.

There are different types of compound microscopes, the most common being a biological microscope, which makes use of bright field microscopy to illuminate the specimen and produce the magnified image.

Final tips

It’s important to remember that while a compound microscope can be magnified hundreds of times, it’s not strong enough to be able to see molecules, viruses, or individual atoms. So, if you need a more high powered microscope, look for a specialized microscope that is specifically designed for your needs and requirements.

Last, when buying a compound light microscope, try to go for the ones with bright LED bulbs, as these are not only brighter but more energy efficient as well. And avoid compound microscopes offering a 2000x magnification, since this is a false magnification and will give you a significantly lower image resolution.