Light microscopes are relatively complex pieces of equipment in nature with multiple different parts, some which are more complex than others.
The lenses of the microscope are fundamental to its function as they provide the magnification power that allows the microscopic specimen to be seen or observed in greater detail. The two main types of lenses found in light microscopes today are called the objective lens and the ocular lens, which is also called the eyepiece.
The ocular lens is positioned at the top of the optical tube, while the objective lens is positioned at the bottom. Both of these lenses have important roles in magnification, but the objective lens also has other defined roles, such as resolving power.
The Function of the Objective Lens in a Microscope
The majority of light microscopes have an objective lens of some kind, including both compound microscopes and stereo microscopes. Both of these types of microscopes also have an eyepiece or ocular lens.
Magnification Power:
The objective lens and the ocular or eyepiece lens are in combination responsible for magnification of the specimen being observed.
For example:
Total magnification = Objective magnification x Ocular magnification.
Therefore, for 10x objective and 10x ocular the total magnification = 10 x 10 = 100x
This means that the specimen being observed is now 100x it’s actual size.
Resolution:
Resolving power is also a very important metric since magnification power is of little importance if the resolution is not high. Resolution is defined as the ability to distinguish 2 points as two points.
For example, if you are looking down a microscope, the resolution power relates to the space you can see between two points. A very low resolution would result in a blurred image and would prevent proper observation of the specimen.
While the total magnification is determined by both the objective and ocular lens, the resolution is determined by the objective lens alone.
Types of Objective Lenses
The majority of compound microscopes come with interchangeable objective lenses, which have different magnification powers. This commonly includes 4x, 10x, 40x, and 100x objective lenses.
Scanning Objective Lens (4x)
Combined with the eyepiece lens, this lens will provide the lowest magnification power. For example, 10x eyepiece lens, multiplied by the 4x objective lens gives a total magnification of 40x.
This objective is often referred to as the scanning objective lens since the low power provides enough magnification to give the observer a good overview of the entire slide and sample.
Low Power Objective (10x)
This objective lens is the next lowest powered and is often the most helpful when it comes to analyzing glass slide samples. The total magnification for this lens is equal to 100x magnification (10x eyepiece lens x the 10x objective equals 100).
Since it still provides a good amount of magnification at a good distance from the slide, there is a limited risk of it breaking the glass and potentially ruining the sample. That’s why this objective lens is often preferred before going for a high powered lens.
High Power Objective Lens (40x)
This is referred to as the high powered objective lens since it is ideal for observing the small details within a specimen sample. The total magnification for this lens is equal to 400x magnification (10x eyepiece lens x the 40x objective equals 400).
Oil Immersion Objective (100x)
This objective lens will achieve the greatest magnification and has a total magnification of 1000x (10x eyepiece lens x the 100x objective equals 1000).
Because glass and air have different refractive indexes, light bends at different angles when it passes through each of them. When using the 4x, 10x, 40x objective lenses, the light refraction that occurs when looking through the lens to the specimen on the glass slide is not very noticeable. However, when using the higher power objective lenses, for example the 100x, the light refraction is much more obvious.
A drop of special oil which has a similar refractive index to glass, is placed on the cover slip over the specimen. The oil immersion objective lens is immersed in the oil, rather than air, enabling a clear image of the specimen.
What is the Difference between Ocular and Objective Lenses?
As previously mentioned, the ocular or eyepiece lens is located at the top of the eyepiece tube and is where you position your eye to observe the specimen. The ocular lens typically has a low magnification (10x) and works in combination with the objective lens to achieve a greater magnification power.
The objective lens is at the bottom of the eyepiece tube and is responsible for both total magnification of the specimen, as well as the resolving power of the microscope.
Cleaning your Microscope Lenses
Often overlooked is the cleanliness of your optics. Daily use in any environment will attract dust and small debris, and when handling your lens, oils from your body can be transferred. This is particularly the case around the eyepiece.
Ensuring your lenses are are kept clean will increase the performance and clarity of your microscope’s images. There are many products on the market but nothing specifically made for microscopes. We found a good quality Digital Camera Cleaning Kit was a great option, something with wipes for removing grease and oils and a puffer bottle for blowing away dust.
Other tips for keeping your equipment clean are:
- Ensure your microscope is packed down after use. If it’s not stored in a case, a dust cover is will help to keep it clean.
- Always wash and dry your hands thoroughly before use. This will decrease contaminates transferred to your equipment.
- Cotton museum gloves can also aid in reducing contamination, however poor quality gloves may leave fibers around. Latex Gloves are also a very good option when handling slides as it will reduce finger prints with no risk of cotton fibers left around to distort images.