How a Compound Microscope Works: Following the Light Path From Lamp to Eye

The Short Answer

A compound microscope magnifies in two stages. The objective lens, close to the specimen, forms an enlarged, upside-down real image inside the microscope tube. The eyepiece then works like a magnifying glass on that image, enlarging it again. Total magnification is the objective’s power multiplied by the eyepiece’s power, so a 40x objective with a 10x eyepiece gives 400x.

That two-lens design is what makes it “compound”. A single lens can only magnify so much before the image becomes hard to use. Splitting the work between two lens systems gives far higher magnification over a short distance.

The rest of this article follows a beam of light from the lamp to your eye and explains what each part does along the way.

The Light Path at a Glance

On a typical upright compound microscope, light travels in this order:

1. Illuminator in the base produces the light.
2. Condenser under the stage gathers it into a cone and focuses it on the specimen.
3. Iris diaphragm in the condenser controls the width of that cone.
4. Specimen on the slide absorbs, bends or scatters some of the light.
5. Objective collects the light and forms the first, magnified image.
6. Body tube or head carries the light upward (and splits it if there are two eyepieces).
7. Eyepiece magnifies the first image a second time.
8. Your eye focuses the result onto your retina.

Every part has a job. If any of them is badly set, the final image suffers, no matter how good the objective is.

Step 1: The Illuminator

Most modern microscopes have a built-in LED or halogen lamp in the base. LEDs run cool and last a long time; halogen gives a warmer color. Older and very basic microscopes use an adjustable mirror to reflect room light or a desk lamp up through the stage instead.

Never point a microscope mirror at the sun. The optics concentrate the light and can seriously damage your eyes.

Better microscopes also have a field diaphragm near the lamp. It sets how wide an area of the slide is lit, and it’s used when setting up Köhler illumination, a standard alignment method that gives even, glare-free lighting.

Step 2: The Condenser and Iris Diaphragm

Light from the lamp spreads out. The condenser, a lens system just below the stage, gathers that light and concentrates it into a cone aimed at the specimen. Many student and hobby microscopes use a simple two-lens design called an Abbe condenser.

The iris diaphragm (also called the aperture diaphragm) sits in the condenser and changes the width of the light cone. This matters more than most beginners expect:

• Opening the iris gives a wider cone of light. This supports the objective’s full resolving power but can wash out pale, unstained specimens.
• Closing the iris increases contrast and depth of field, but closing it too far reduces resolution and can add dark fringes around edges.

The iris is a contrast and resolution control, not mainly a brightness control. Adjust brightness with the lamp dimmer where you can, and use the iris to balance contrast against detail. A common starting point is to open it until the image just starts to lose contrast, then close it slightly.

Raising or lowering the condenser also changes the lighting. On most microscopes it works best near the top of its travel, just below the slide.

Step 3: Through the Specimen

Compound microscopes use transmitted light: it passes up through the specimen rather than bouncing off it. That’s why specimens need to be thin and at least partly transparent, such as a drop of pond water, a single layer of onion skin or a stained tissue section.

As light passes through, parts of the specimen absorb some of it (which is what stains exploit) or bend it slightly. Those small differences are what the objective turns into an image. Very transparent specimens change the light so little that they’re hard to see in ordinary brightfield, which is why techniques such as darkfield and phase contrast were developed. Our guide to the 7 types of light microscopes covers these.

Most specimens sit under a thin coverslip. Many objectives are designed for coverslips about 0.17 mm thick, and the coverslip forms part of the optical system at higher powers.

Step 4: The Objective Forms the First Image

The objective is the most important optical part of the microscope. It sits just above the specimen, and most microscopes carry three or four on a rotating nosepiece, commonly 4x, 10x, 40x and 100x.

When you focus, the specimen ends up a little farther from the objective than the objective’s focal length. Under those conditions, a converging lens forms a real, inverted, magnified image farther along the light path. In a microscope, that image forms inside the tube, near the bottom of the eyepiece. It’s called the intermediate image. Our explainer on real vs virtual images in a microscope goes into what “real” means here, and how lenses magnify covers the refraction behind it.

Each objective is engraved with numbers. The two most useful are:

• Magnification, such as 10x or 40x.
• Numerical aperture (NA), such as 0.25 or 0.65. NA describes how wide a cone of light the objective can collect. It largely sets the finest detail the objective can resolve. Higher NA means better resolution.

The 100x objective is usually an oil-immersion lens. A drop of immersion oil between the lens and the coverslip lets it collect light that would otherwise be lost by bending at the glass-air boundary, which raises its NA and resolution.

Higher-power objectives also have a shorter working distance, the gap between the lens and the coverslip when in focus. At 40x and 100x it’s small, which is why you focus carefully at high power.

Step 5: Up the Tube

What happens between the objective and the eyepiece depends on the design.

• Finite tube length. Many older and basic microscopes are designed so the objective forms its image a set distance up the tube. A mechanical tube length of 160 mm is the most common standard for these. Objectives made for 160 mm systems are often marked “160”.
• Infinity-corrected. Most modern research microscopes and many newer mid-range ones use infinity-corrected objectives, marked with the infinity symbol (∞). The light leaving the objective comes out as parallel bundles, and a separate tube lens in the body brings it to a focus to form the intermediate image. The parallel section lets manufacturers insert filters, prisms and other parts without upsetting the focus.

The two systems aren’t interchangeable. Finite and infinity objectives should only be used on microscopes designed for them, and infinity objectives from different makers aren’t always compatible with each other either.

Binocular and Trinocular Heads

On a microscope with two eyepieces, prisms in the head split the light into two paths, one for each eye. A trinocular head adds a third path for a camera. Each eye sees the same image from the same objective, so a binocular compound microscope is more comfortable to use but doesn’t give a true 3D view. A stereo microscope, which has separate optical paths for each eye, does.

Step 6: The Eyepiece Magnifies Again

The intermediate image lands at the field stop, a fixed ring inside the eyepiece. The field stop gives the circular edge you see when you look into the microscope. Because it sits in the same plane as the intermediate image, anything placed there, such as a measuring scale (reticle), appears sharp along with the specimen.

The eyepiece then acts like a magnifying glass looking at that intermediate image. Most eyepieces are 10x, though 5x to 20x are common. The eyepiece doesn’t form an image you could project on a screen. Instead, it sends out light that your eye interprets as a large virtual image that appears to be some distance away. This is why you can view a microscope image with your eye relaxed, as if looking at something across the room.

Step 7: Into Your Eye

Above the eyepiece, the light converges to a small bright disc called the exit pupil or eyepoint. Your eye’s pupil needs to be at that point to see the whole field of view. The distance from the eyepiece lens to the eyepoint is the eye relief. If you hold your eye too far away or too close, you’ll see only part of the field or a dark ring.

Your eye’s own lens then focuses the light onto your retina, forming the final real image that you perceive. In that sense, a compound microscope is a three-lens system: objective, eyepiece and eye.

Eyepieces marked with a glasses symbol, or described as “high eyepoint”, have longer eye relief. They let people who wear glasses keep them on, which is helpful for astigmatism. People with only near- or long-sightedness can usually take their glasses off and refocus the microscope instead.

Why the Image Is Upside Down and Backwards

The objective forms an inverted real image, flipped top to bottom and left to right. The eyepiece magnifies that image without flipping it back. So what you see is rotated 180 degrees compared with the specimen.

In practice, this means that when you push the slide to the left, the image moves to the right, and when you push it away from you, the image moves toward you. It feels awkward at first but becomes second nature within a few sessions. A mechanical stage with control knobs makes it easier.

Stereo microscopes include prisms that turn the image the right way round, which is one reason they’re preferred for dissection and small repairs.

Working Out Total Magnification (and Its Limits)

Total magnification = objective magnification × eyepiece magnification.

Some microscopes have an extra lens in the head that adds its own factor, such as 1.25x or 1.5x. If so, multiply that in as well. The manual usually mentions it.

Magnification has a useful ceiling. A common rule of thumb puts the range of useful magnification at roughly 500 to 1000 times the objective’s numerical aperture. A 100x objective with an NA of 1.25, for example, reaches its useful maximum somewhere around 1250x. Beyond that you get empty magnification: a bigger image with no extra detail. That’s why a microscope advertised at 2000x or more with ordinary objectives is usually a warning sign rather than a selling point.

Using the Light Path to Fix Common Problems

Knowing the order of the parts helps you track down what’s wrong.

Compound vs Simple and Stereo Microscopes

A compound microscope is one member of a larger family:

• A simple microscope uses a single lens, such as a magnifying glass. See how a simple microscope works for how one lens magnifies on its own.
• A stereo microscope uses two separate optical paths at low magnification to give a 3D view of solid objects.
• Electron microscopes replace light and glass with electrons and magnetic lenses. Our comparison of light vs electron microscopes explains the trade-offs.

For an overview of the instrument families and key terms, start with our introduction to microscopy. Once you understand the light path, setting up lighting, choosing objectives and fixing a poor image all become much easier.