In light microscopy, a water immersion objective is a specially designed objective lens

Objective (optics)

In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 used to increase the resolution

Optical resolution

Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...

 of the microscope. This is achieved by immersing both the lens and the specimen in water which has a higher refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 than air, thereby increasing the numerical aperture

Numerical aperture

In optics, the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one...

 of the objective lens.

Applications

Water immersion objectives are used not only at very large magnifications that require high resolving power, but also of moderate power as there are water immersion objectives as low as 4X. Objectives with high power magnification have short focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

s, facilitating the use of water. The water is applied to the specimen (conventional microscope), and the stage is raised, immersing the objective in water. Sometimes with water dipping objectives, the objective is directly immersed in the solution of water which contains the specimens to look at. Electrophoretic preparations used for instance in the cases of Comet Essay can benefit largely with water objectives.

The refractive indices of the water and of the glass in the first lens are different but less than it would be the cases between air and glass as it will be the case with a non immersion objective, which means that the refraction of light will be small upon entering the lens.

Correction collar

Unlike oil, water does not have the same or near identitical refractive value as the cover slip glass, so a correction collar is needed to be able to variate for its thickness. Lens without correction collar generally are made for the use of a 0.17 mm cover slip or for use without a coverslip (dipping lens).

See also

• Oil immersion objective
• Microscopy
  Microscopy
  Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
  
  
• Optical microscope
  Optical microscope
  The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
  
  
• Index-matching material
  Index-matching material
  In optics and fiber optics, an index-matching material is a substance, usually a liquid, cement , or gel, which has an index of refraction that closely approximates that of an optical element or fiber, and is used to reduce Fresnel reflection at the surface of the element.In fiber optics and...