Water immersion objective
In light microscopy, a water immersion objective is a specially designed objective lens used to increase the resolution of the microscope. This is achieved by immersing both the lens and the specimen in water which has a higher refractive index than air, thereby increasing the numerical aperture of the objective lens.
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 lengths, 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.
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).