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An oil immersion lens is a microscope objective, usually the 100x, designed to be used with a drop of immersion oil between the front lens and the specimen cover glass. The oil has a refractive index close to that of glass (nominally 1.515), so light bends less on its way into the objective and more of it is collected. This page is general equipment information, not instructions for a clinical procedure or medical advice.
Definition and scope
“Oil immersion lens” is the everyday name for an oil immersion objective: one engraved for use with oil rather than in air. The Motic guide to immersion oil notes that such objectives are marked “oil”, “immersion” or “HI” (homogeneous immersion), and that oil should not be used on objectives without that marking. The oil is a separate consumable, which is why “immersion oil for microscopy” is searched alongside the lens.
What is an oil immersion lens and how does it work?
Air has a refractive index of about 1.0 and glass about 1.5. Without oil, light leaving the cover glass meets an air gap and is bent away from the objective, so part of it never enters the lens. Replacing the air with oil whose index is close to the glass lets more light reach the objective (Motic, read 2026-11-07). For the numerical aperture ceilings and how to pick between objective classes, see microscope objective selection; for the resulting limit on detail, see the optical microscopy resolution limit; and for the lens train around it, compound microscope optics. Some dark-field set-ups also oil the condenser, covered in dark-field microscopy.
What is the refractive index of immersion oil?
For Cargille Type A immersion oil the datasheet gives n = 1.5150, but only at a stated condition: 589.3 nm and 23 °C. The number is nominal, because the index changes with wavelength and temperature.
| Wavelength (Type A, 23 °C) | Refractive index |
|---|---|
| 486.1 nm | 1.5239 |
| 546.1 nm | 1.5180 |
| 589.3 nm | 1.5150 |
| 656.3 nm | 1.5114 |
The same datasheet lists an Abbe number (vD) of 41.2 and a temperature coefficient (dn/dT, 15 to 35 °C) of -0.00033 per °C, meaning the index falls as the oil warms. Source: Cargille Laboratories, Immersion Oil Type A datasheet, dated 18 July 2018 (cargille.com/immersion-oil-type-a), read 2026-11-07.
What is the difference between immersion oil Type A and Type B?
On Cargille datasheets the two differ in viscosity, not refractive index. Both list n = 1.5150 at 589.3 nm and 23 °C and the same composition, density and flash point.
| Property (Cargille datasheets, 18 July 2018) | Type A | Type B |
|---|---|---|
| Refractive index, 589.3 nm, 23 °C | 1.5150 | 1.5150 |
| Viscosity at 23 °C | 150 cSt (138 cP) | 1250 cSt (1153 cP) |
| Density | 0.923 g/cc | 0.923 g/cc |
| Flash point | approx. 163 °C | approx. 163 °C |
Type B is therefore roughly eight times as viscous as Type A. These are one manufacturer’s designations; other suppliers may define their types differently, so read the datasheet for the oil you actually hold.
What are the different types of immersion oils used in microscopy?
The Cargille datasheets read for this page describe Type A and Type B, both made of aliphatic and alicyclic hydrocarbons, polybutenes and hydrogenated terphenyls, colourless to slightly yellow with a slight odour. Speciality oils sold for other uses are described on their own suppliers’ datasheets, which are the authority for those.
Where it matters for labs: handling, cleaning and compatibility
- Check the objective marking (“oil”, “immersion” or “HI”) before putting oil on any lens (Motic).
- Cleaning: Motic advises lens paper first, then paper moistened with ethyl or isopropyl alcohol or a lens-cleaning solution; for hardened oil, xylene or lens-cleaning solution left to soften it. Residue affects later image clarity.
- Solvents: Cargille lists the oil as soluble in xylene, heptane and toluene and insoluble in ethanol and water, and gives acetone with a wipe for cleaning it off glass. It lists polystyrene among incompatible materials, so keep oil away from polystyrene parts and plasticware.
- Storage: the oil is noted to darken slightly in direct sunlight.
Follow the objective maker’s own cleaning instructions where they differ, since coatings and cements vary by manufacturer.
Common misunderstandings
- “1.515 is the exact index.” It is the value at 589.3 nm and 23 °C; the table above shows it moves with wavelength.
- “Type B is a higher-index oil.” On Cargille datasheets the index is identical; viscosity differs.
- “Any 100x objective takes oil.” Only objectives marked for oil or immersion do.
- “Ethanol will dissolve oil residue.” The datasheet lists the oil as insoluble in ethanol; Motic recommends alcohol only on paper after lens paper has removed the bulk.
FAQ
What is an oil immersion lens?
A microscope objective, typically 100x, marked for use with immersion oil between lens and cover glass so that more light is collected than through air.
What are the different types of immersion oils used in microscopy?
Cargille Type A (150 cSt) and Type B (1250 cSt) share the same nominal index of 1.5150 and composition; they differ in viscosity.
What is the refractive index of immersion oil?
1.5150 for Cargille Type A and B at 589.3 nm and 23 °C, falling to 1.5114 at 656.3 nm and rising to 1.5239 at 486.1 nm.
What is the difference between immersion oil Type A and Type B?
Viscosity: 150 cSt against 1250 cSt at 23 °C, with identical index, density and flash point on the datasheets.
Sources
- Cargille Laboratories, Immersion Oil Type A datasheet (18 July 2018), https://www.cargille.com/immersion-oil-type-a/, read 2026-11-07.
- Cargille Laboratories, Immersion Oil Type B datasheet (18 July 2018), https://www.cargille.com/immersion-oil-type-b/, read 2026-11-07.
- Motic, Using Microscope Immersion Oil, https://www.motic-microscope.com/using-microscope-immersion-oil, read 2026-11-07.
