“Like what you are used to seeing in a lab on television or in movies, a microscope that views slides with a bright background. There are many types of compound microscopes, for more specific applications, however in general a compound microscope is a high power microscope meant to view things that the naked eye cannot see unaided.”
A compound microscope is likely to be the first kind of microscope anyone uses in school. Most junior high schools or high schools do a lab in which they inspect their cheek cells or skin cells under a microscope–usually with up to 400x magnification. I know I did, and my younger brother did too. I don’t expect that to change anytime soon either.
So what is a compound microscope?
A compound microscope is a device in which two convex lenses are used to create magnification of a sample too small to see with the human eye. It is commonly considered a “high power” microscope because of it’s ability to magnify tiny elements with great detail and resolution, and allowed many modern advances in science and medicine. Cells and bacteria are visible with this type of a microscope, and any other microscopic organism or mono-cellular structure (single cell organisms), or even more!
How does a compound microscope work?
The best way to talk about how a compound microscope works is to follow the path of light from the light source to the eye (or camera). On a standard compound microscope, light is emitted from the bottom of the unit. This can be from a mirror or a light bulb of some kind–doesn’t really matter as long as it’s bright enough. From here, light is directed up towards the stage, where the sample sits. Between the sample and the light source, there are a few components. First of all, most microscopes (at least those of value) come with a condenser of some kind. A condenser’s job is simply to take the light and focus it with better precision on to the sample on the stage. Otherwise, you’d just get this big cone of light, and a lot of it gets wasted. The condenser can have an adjustable iris or sometimes even varying hole sizes on a simple disc, to control how the light is being focused–remember, we don’t want to waste light, we want it to get focused on the sample. Once the condenser passes the light on to the sample, the sample is visible. We’ve made it about half way through the system.
The light has to travel through the sample in order for the sample to be visible. This is why rocks, coins, and stamps cannot be viewed with this. If the sample is solid, and all light is blocked, there’s nothing to see–you’ll simply get a black silhouette of the sample, if light can travel around it, or a completely black field of view if the sample is too large for any light to go around. Now, in order to actually enlarge the image, that light (which now carries the color data from the sample it passed through) has to enter the objective lens. This lens is the bottom lens that points to the sample, and usually comes in a few different powers on a microscope (4x, 10x, 40x, and sometimes 100x are common objectives on a compound microscope). From here, the objective passes the light on to the prism in the microscope head, which reflects the light up towards the ocular tubes. The eyepieces then gather the image, and further magnify it for your eyes to see clearly.
Why is the image in the compound microscope inverted?
The image in a microscope is inverted because the each time the light passes through a lens, the direction of the image is flipped. After all of the flipping at each lens in the microscope, it ends up inverted from the original, which is why when you move a sample left on the compound microscope stage, it moves right instead.
In a compound microscope, the eyepiece only serves to magnify the image provided by the objective lens. The objective lens is the front lens, and is the only lens the light travels through after passing through the sample.
Some advanced microscopes can use a projector lens that reverts the image back upright, but tend to be extremely high in cost.
For the visual learners, this image shows the path of light in a compound microscope, helping to explain the above.
Are there different kinds of compound microscopes?
Absolutely! Compound microscopes are most commonly used for “brightfield” microscopy, which basically means that the path of light looks as pictured above, and the sample is shown against a bright background (or a bright field of view). Some samples don’t contrast well in brightfield, or sometimes you need to see other parts of your sample with greater contrast than the general image. Blood cells are certainly visible in brightfield, but what if you wanted to check the cell shape for sickle cell disease? It can be a bit tricky to see since it’s such a bright background the cells are contrasted against. To do this, you can use darkfield microscopy instead. Similar to this is phase contrast. If you have a clear sample, it’s not going to contrast well against a bright background. It will appear as if you’re looking right through the sample. You might see some slight aberrations of light where it’s thicker, but in general you won’t see much detail. This is where phase contrast comes in. It’s quite technical in nature, and will be fully explained in its own section, but essentially what this does is adjusts where in the wavelength of light it hits the eye. By altering this, the appearance of the light is changed quite drastically, and provides a great deal of contrast in transparent samples. Another reason to use this is when inspecting live specimen that are translucent–dying them will kill them off, so this is used for doing waste water treatment and urology samples most commonly. For more information on each, you can visit their full details pages here:
There are also a few other kinds of compound microscopy that do not change the core essence of the unit by changing a condenser, and these can be found on the bottom index of the Microscope Basics page (click below to travel back).