Why Using Immersion Oil with a 100x Objective Lens is Essential

Video why do you use immersion oil with 100x objective lens

Have you ever wondered why scientists and researchers use immersion oil with a 100x objective lens when observing microscopic samples? The answer lies in the way light behaves when it passes through different materials. In this article, we will explore the significance of using immersion oil and how it enhances the quality of microscopic images.

Understanding Light Refraction

When light passes from one material to another, it bends or refracts. This phenomenon is especially apparent when light transitions from glass to air. The refractive index of air is around 1.0, while the refractive index of glass is approximately 1.5. As light passes through the space between the microscope objective lens and the slide (which is filled with air), it scatters and loses intensity. Consequently, as magnification increases, images become less distinct due to the bending of light at different wavelengths.

The Role of Immersion Oil

Although the refraction of light is usually inconspicuous when using lower magnification objective lenses (such as 4x, 10x, and 40x), it becomes noticeable with the 100x objective lens. To minimize light refraction and maximize the amount of light passing through the microscope slide, a substance with a refractive index similar to that of glass is necessary. This is where immersion oil comes in.

By placing a drop of immersion oil with a refractive index matching that of glass on the slide, more light can be directed through the objective lens, resulting in clearer and crisper images. Take a look at the images below, captured using the UX1 microscope with both the 100x Achromat and 100x Plan Achromat objective lenses. Notice the striking difference in image quality between the dry and immersion oil methods.

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Microscopy image of duodenum

Using Immersion Oil Correctly

To ensure optimal results, it is crucial to follow the proper steps when using immersion oil with a 100x objective lens:

  1. Begin by focusing your sample using the 40x objective lens. Partially rotate the objective lens between 40x and 100x to access the cover slip on your slide.
  2. Place a drop of immersion oil on the cover slip and another drop directly on your 100x oil objective lens.
  3. Slowly rotate the 100x oil objective lens into place and adjust the fine focus until you achieve a crisp and clear image.
  4. After viewing with the 100x oil immersion lens, carefully wipe the oil from all glass surfaces using lens cleaning paper.
  5. Use a second piece of lens paper moistened with a small amount of alcohol or lens cleaning solution to remove any remaining oil.
  6. In case immersion oil hardens on the lens, moisten lens paper with xylene or microscope lens cleaning solution. Gently wipe the lens surfaces.

When to Use Immersion Oil

It is important to note that immersion oil should only be used with lenses specifically designed for it, usually labeled as “oil,” “immersion,” or “HI” (homogeneous immersion). Immersion oil is most effective when viewing samples that are non-moving and no thicker than a few micrometers. It is particularly useful for observing individual bacteria strands or discerning details of striations in skeletal muscle. If you aim to obtain a clearer image at 1000x magnification, using immersion oil is a must.

However, there are a few drawbacks to consider. To use immersion oil with a wet mount slide, you need to ensure its secure placement. Additionally, when switching between a dry 40x lens and a 100x oil lens, caution must be exercised to avoid damaging the 40x lens with oil.

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For any inquiries regarding the use of immersion oil with your microscope, feel free to contact Microscope World. They offer a wide range of microscope immersion oils to suit your needs.


Understanding why immersion oil is crucial when using a 100x objective lens is essential for obtaining clear and high-quality microscopic images. By reducing light refraction, immersion oil allows more light to pass through the slide, resulting in improved visibility of minute details. Remember to follow the correct procedure for using immersion oil and enjoy exploring the fascinating microscopic world with enhanced clarity!

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