What does oil do for oil immersion lens?

• A. Reduces refraction and improves resolution at high magnification
• B. Helps clean the lens
• C. Cools the slide
• D. Provides color contrast

Answer: (A)

Oil immersion works by using a special oil whose refractive index is very close to glass to fill the space between the slide and the objective lens. This index matching greatly reduces light refraction at the glass–oil–air boundaries, so more light from the sample enters the lens. With less bending and a higher effective numerical aperture, you get finer detail and sharper images at high magnification. The oil’s role is not to clean the lens, cool the slide, or create color contrast—the main benefit is improved light collection and resolution through refractive index matching.