Microscopes have been an essential part of the scientific world to help researchers study the world around them. From hand-held microscopes to compound light microscopes and electron variants, microscopes have come a long way. The objective lenses hold significant importance among the different parts of a microscope; they are critical for observation and image formation that you see in the eyepiece. This article covers everything about objective lenses, including the types and functions of objectives in a microscope.
What are Objective Lenses?
Objective lenses are present on the bottom side of the nosepiece just above the stage. They appear as small cylindrical objects attached to a rotatable disk.
A few types of microscopes have three lenses, while others have an oil immersion lens of high magnification, making up a total of four objective lenses.
Objective lenses are among the most important elements of a microscope that are easily switched for better magnification. Without objective lenses, we would be unable to observe the specimen clearly.
So, what is the function of objectives in a microscope?
Function of Objective Lens in a Microscope
The main function of objectives in a microscope is to gather light from the illumination source to form an image of the specimen.
The light from the illumination source passes through the sample to the objective lens, which produces an image of the sample. This image then shows in the eyepiece or is projected onto a screen, such as in digital microscopes.
If you wonder about the function of objectives in microscope scanning, the primary focus is the same – producing images. The objective lenses in scanning microscopes focus the illumination to a small point to display the sample. The point of illumination-sample interaction is point-by-pointe detected to form an image.
Magnification of Objective Lenses
Magnification refers to the ability of a microscope to enlarge the image beyond its real size. It is the most important feature of microscopy to study microorganisms and micro-objects.
The most common 3 types of objectives in microscopes are 4x, 10x, and 40x which work with eyepiece magnification lenses to produce images. Some microscopes have 4x, 10x, 40x, and 100x objectives. These lenses are scanning, low power, high power, and oil immersion objectives respectively; color bands also indicate different magnification powers.
High magnification lenses, including 40x and 100x, are spring lenses that help protect the lenses from damage. Some advanced microscopes also have 200x objective lenses.
| Objective Lens Magnification | Objective Lens Type | Band Color | Usual Specification |
| 4x | Scanning | Red | Rigid |
| 10x | Low-power | Yellow | Rigid |
| 40x | High-power | Light blue | Spring |
| 100x | Oil immersion | White | Spring |
Numerical Aperture of Objective Lens
As the main function of objectives in a microscope is to magnify the specimen and create an image, the numerical aperture is another critical element. It is the function of the entrance pupil diameter and the focal length.
Objective lenses with high numerical apertures require immersion oil between the objective and sample. Immersion oil helps change the refractive index to produce a better specimen image.
Field of View
Field of view refers to the whole area of the stage observed in a microscope. The objective lens captures the field according to the objective lens magnification. The enlarged image is then further magnified by the eyepiece lens. Thus, a higher magnification lens provides a smaller field of view. The FOV also depends on the shape of the display object. For example:
- Standard compound microscopes provide the field of view after the eyepiece further magnifies the observation
- Microscopes with a screen (typically rectangular) only show some part of the FOV due to changes in shape and dimensions
- The eye retina shows the complete FOV from the objective lens as it has the same (circular) shape
Types of Objectives in Microscope
Refractive Objective Lenses
Microscopes usually have refractive objective lenses that bend the light coming from the sample. Furthermore, the anti-reflection coating helps avoid back reflections. Refractive objective lenses come in various types, with two elements in basic achromatic lenses, fifteen in plan-apochromatic objectives, and more.
Reflective Objective Lenses
Unlike refractive objective lenses, reflective ones use a mirror-based design with a primary and a secondary mirror. Reflective objective lenses offer higher light efficiency compared to refractive glass lenses. So, they do not require additional elements to fix the aberrations otherwise. At the same time, these objective lenses offer better resolution.
Reflective objective lenses also allow you to study objects under the IR and UV regions as they do not use glass to study the specimen.
Corrected Objectives
As the common use of objectives in microscope involves refractive lenses, they require corrections to provide high-quality images of the specimen. Most laboratory microscopes use achromatic objectives. They are corrected for axial chromatic aberration in red and blue wavelengths by bringing them to a common point. Furthermore, these lenses also have spherical aberration correction in green color to produce magnified, color-corrected images.
Alternatively, fluorite and apochromatic lenses possess higher levels of correction. Flourite objective lenses are made with glass formulations containing fluorspar. These lenses are corrected for blue and red chromatically and corrected for up to three colors spherically. Flourite lenses have a higher resolution and better contrast.
Lastly, the apochromatic objective lenses are the most expensive and have the highest level of correction. These lenses are corrected for three colors, so they have a higher level of numerical aperture. Advanced lenses are corrected for four colors spherically and four or more colors chromatically.
Applications of Objective Lenses
The use of objectives in a microscope is versatile, but they are not found in microscopes only. They are a component of various scientific, industrial, and medical instruments, including cameras, binoculars, telescopes, etc. Every application uses different types of objective lenses. However, objectives eventually contribute to the imaging of the sample.
The Bottom Line
Objective lenses are among the most important parts of a microscope as they mainly determine the magnification of the sample. The main function of objectives in a microscope is to gather the light from the specimen and form an image. Objective lenses come in 2x to 200x magnifications; 4x, 10x, 40x, and 100x are the most common. Most microscopes use refractive objective lenses, whereas some also have reflective lenses. Objective lenses are found in microscopes, telescopes, cameras, binoculars, and similar instruments.
FAQs
What are the objective powers of a microscope?
Objective powers refer to the magnification of the objective lenses in a microscope. Most microscopes have objectives of 4x, 10x, 40x, and 100x. However, you can find 2x to 200x power objective lenses in different pieces of equipment.
What is the function and use of low power objective in microscope?
Lower power objectives (10x objective lenses) are suitable for large samples or multiple smaller specimens. They cover a wide field of view as these lenses do not enlarge the sample too much.
Which objective gives a larger image?
Lower magnification objectives like scanning and lower power lenses offer a large field of view. However, they do not magnify the specimen as much as high-power objectives. So, a high magnification objective is more suitable if you want to see a larger image of the primary object.
Which objective lens should you use first for viewing a slide and why?
Always start with the low-power objective lenses to set the microscope, eventually moving to high-power lenses. It also ensures that you do not damage high-power lenses while adjusting the stage initially.
Which objective should be in place when the microscope is put away?
The length of objective lenses increases with the increase in magnification. So, a 4x objective in a microscope is much shorter than a 100x objective lens. When storing the microscope, you must place the smallest (4x) objective over the stage to avoid damage.
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