Why is a Specimen Less Than 200 nm Not Visible With a Light Microscope?

Microscopes are critical to observing the in-depth structure of microscopic organisms around us. While optical and electron microscopes both contribute to the visibility of an organism, most students and researchers use light microscopes. However, the light microscope has its own limitations. For example, it cannot observe less than 200 nm. If you wonder why, you’re at the right place. This article focuses on why is a specimen less than 200 nm not visible with a light microscope and how to observe smaller samples.

Keep reading to know why the light microscope does not show a specimen under 200 nm.

Why is a Specimen Less Than 200 nm Not Visible with a Light Microscope?

Even if you have observed a specimen under the best light compound microscope, you might not have been able to see samples less than 200 nm.

Specimens less than 200 nm are not visible with a light microscope, as anything smaller than 200nm cannot interact with visible light.

Visible light only works and shows specimens up to wavelengths of 390 nm. Thus, we cannot see anything under 200 nm under a light microscope. We have explained this further for your easy understanding.

Specimen Less Than 200 nm Not Visible with a Light Microscope Explained

An English physicist, Lord Rayleigh, showed that the smallest object observable under a microscope depends on size of the lens, type of microscope lens, and the wavelength. He shared his studies in 1879 regarding the limitation of lens and light towards allowing observation of the sample.

According to his research, optical microscopes use about half the wavelength of the light used. These microscopes typically use blue light with a wavelength of 400nm, which is why the smallest observable object is around 200 nm.

The process is also explained using diffraction. The diffraction patterns of different observations cannot overlap more than a specific extent and cannot be avoided either. Visible light produces a diffraction pattern when it passes through a slit or whole. The same happens when the condenser converges the light beam from the illumination source.

Thus, the light on the slide comprises multiple concentric circles, limiting the resolution of the specimen.

How Does Resolution Limit the Magnification of a Light Microscope?

Light microscopes with combined lenses have the ability to magnify objects to a large extent, but the image would have low resolution, making it blurry. Resolution is the property of an image that lets you distinguish between two points, giving you crisp images. Thus, it contributes to the clarity of the observation.

Now, as we mentioned, the observation of a specimen depends on the wavelength as it influences the resolution. The object under observation might not be visible under the light microscope if the sample is too smaller than the wavelength of the illumination source. Considering that the wavelength of the light is 400 nm, and half of it is 200 nm, the microscope cannot observe objects less than 200 nm.

How can We Observe Specimen Less than 200 nm?

As you cannot view a specimen less than 200 nm with a light microscope, the best choice is to use an electron microscope.

The inability of light microscopes to view objects smaller than 200 nm is attributed to the wavelength of visible light; electron microscopes eliminate this limitation.

Electron beams in these microscopes have a wavelength 100,000 times shorter than visible light. Thus, you can observe smaller objects in an electron microscope without damaging resolution.

The Bottom Line

Light microscopes are excellent for observing microbes and other samples around us, invisible to the naked eye. However, specimens under 200 nm are not visible with a light microscope. The answer to why is a specimen less than 200 nm not visible with a light microscope is attributed to the wavelength of light, which is 400 nm. Eventually, the microscope cannot view objects much smaller than the wavelength of light. At the same time, electron microscopes may have a wavelength of up to 100,000 times shorter than visible light. Thus, you can see smaller samples in an electron microscope.