Why is the oil immersion objective used to examine bacterial cells (rather than 40x or 10x)?
Lenses can provide huge magnification, but what you need is useful magnification. You should experiment with other lenses, if you get a chance. Use the lowest magnification that will provide the information to your eyes. That will give the highest quality image for the lenses you are using. An oil immersion lens provides what I call optical coupling. The oil used has the same refractive index as the glass used in the lens. Iif you used a lens without oil, the light would face two surfaces where extra losses of information would occur: a) as the light leaves the bacterial sample b) as the light enters the lens. This does not happen with oil immersion lenses. If you compare like with like, a conventional non oil immersion lens of the same magnification would be very inferior. Oil immersion lenses are expensive, but they are far superior at high magnifications, i.e. the image is much clearer.
