Transmitted light microscopes explained

Most optical microscopes you’ll be familiar with are known as ‘transmitted light’ microscopes.

The ‘transmitted’ part of the name comes because light is shone or ‘transmitted’ through your sample to give you a better look at it. This contrasts with ‘reflected light’ that bounces off the sample instead of passing through it.

Transmitted light microscopes make it possible to study almost anything in its natural colours, thanks to the reflections that occur. They’re commonly used to reveal the cellular structures of microorganisms, tissue and crystalline minerals.

You won’t be able to observe every structure of some living things because of the natural limitations of light’s wavelengths, but you will get a good sense of what the cellular components of your sample are when using this equipment.

How they work

The basic principle is ages old – shine light through your sample to get a good look at its structure and characteristics. Modern microscopes follow the same principle but take it a step or two further.

Simpler techniques focus the light using a condenser with an adjustable aperture, giving you better control over contrast and resolution. More advanced techniques go further still, using special filters or lenses to help visualise normally invisible specimens, structures or optical qualities.

There is a whole range of transmitted light microscopy techniques and designs, each with their own advantages and specialised uses. Despite their differences, they are all still based on the principle of shining light through a sample.  

Types of transmitted light microscopy

Brightfield

A brightfiled microscope is the most basic form of transmitted light microscopy. Your samples partially block the light shining through them, showing up as darker images on a brightly illuminated backdrop.

It can often be difficult to see transparent samples on such a bright background, so samples are often stained with colourful dyes beforehand so they’re easier to see.

Darkfield

Darkfield microscopy is a slightly more advanced technique. Instead of a dark image on a bright background, it produces a bright image against a dark background.

Light is still transmitted through the sample, but a dark disc is placed in front of the light source so that only a thin outer halo of light gets past. As the light is now coming in at an oblique angle, it would ordinarily not enter the lens and you would only see the black background of the dark disc.

But when the angled light hits your sample, it is refracted and reflected into the lens and you’ll see your sample brightly lit against the dark backdrop.

This technique is commonly used for specimens with naturally low contrast and is a popular choice for a variety of biological samples.

Polarized Light

Polarized light microscopy uses filters to only allow light of a certain orientation to pass through the sample.

This approach is primarily used on birefringent samples – substances that have two different refractive indexes, depending on the polarization of the light passing through it. Light passing through can come out the other side in two different orientations.

By passing polarized light through the birefringent sample and then through a second, adjustable polarizer, you can selectively filter either of the differently oriented light waves to see different aspects of the sample.

This approach is very common in geology as it’s very helpful when analysing rock and mineral samples, but it’s also used in biological fields and materials sciences.

Phase contrast

This technique is useful for making transparent samples and specimens more visible.

When light passes through transparent samples, it is very slightly refracted. Phase contrast microscopes take that normally invisible change in the phase of the light and convert it into visible changes in brightness.

The increased contrast makes it much easier to see important details, such as internal components and cellular structures. This approach also has the advantage of not harming or damaging specimens in any way, so it’s an excellent choice for looking at live samples.

DIC

Differential interference contrast (DIC) microscopy is another method of looking at transparent samples that are difficult to see.

Light is polarized, then split into two separate parallel beams that pass through the sample. The beams are altered and refracted by the contents of the sample, then recombined back into a single beam once again.

The interference between the now-different wavelengths of the two beams brightens or darkens the image, producing a more visible image with much better contrast.