What is the correct immersion oil practice for the 100X objective?

• A. Immersion oil is never used at 100X
• B. Immersion oil is used only at 40X
• C. Immersion oil is optional at 100X
• D. Oil immersion should be used at 100X

Answer: (D)

Oil immersion at 100X is used to maximize resolution by matching the refractive index between the coverslip, immersion oil, and the objective lens. The 100X objective is designed for immersion oil, which fills the tiny space between the slide and the lens and eliminates the air gap that would otherwise refract light away. This keeps more light entering the objective and preserves a higher numerical aperture, yielding a brighter, sharper image with finer detail. Without oil, brightness and resolution drop significantly, and using oil only at lower magnifications doesn’t take advantage of the objective’s design. So, the best practice is to apply immersion oil when using the 100X objective. For proper technique, place a small drop of oil on the slide, bring the objective in gently until it contacts the oil, then focus. Afterward, wipe any residual oil from the lens and slide to prevent buildup.