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Affordable Microscopes for Everyday Use

May 20, 2013

Immersion Oil Explained

We are often asked about immersion oil so here is a basic primer. Immersion oil is used with high power objectives, typically 90x or 100x.

Light microscopes have an upper limit to their resolving power of marginally over 1,000x. At this level of magnification, the microscope needs to direct every available amount of light in order to achieve a clear image. Since light is refracted and scattered in the air between the objective lens and the slide cover, immersion oil is used to capture much of that ‘lost light’.

In summary, light refracts through air and glass at different angles. The refractive index of air is 1.0 and that of glass, 1.5 so there is considerable refraction between the two. The immersion oil helps to reduce the refraction since it has a refractive index equal to glass. As a result, it forms a continuum between the objective lens and the slide, thereby successfully ensuring that more light is directed towards the specimen and ultimately, a clearer image.

Oil immersion objective lenses are typically engraved with the word “oil”, “immersion” or “HI” (homogenous immersion). They are manufactured with sealants to prevent damage from the oil.
Immersion oils are commonly available in two viscosities-low viscosity (Type A), and high viscosity (Type B). They are often labeled with a refractive index of 1.515. The low viscosity oil is applied to the airspace between slide and objective, the high viscosity oil is applied between the condenser and the slide.

How to Use it: Type A – Low Viscosity Oil

The majority of applications require Type A oil, which can be used as follows:

  1. Locate a specimen on the slide and center it in the image field.
  2. Rotate the nosepiece until the 100x objective lens is just to one side of the slide. Place a single drop of immersion oil on the slide cover slip and place a drop directly on the objective lens. Combined, both drops ensure no air is trapped in between.
  3. Rotate the 100x objective into place and adjust the fine focus to fully resolve the image.


It is very important to carefully clean the oil off your objective lens before it dries.

  1. Carefully wipe oil from all glass surfaces with a folded piece of clean lens paper.
  2. Moisten a piece of lens cleaning paper with lens cleaning fluid and wipe away any residual streaks of oil.