Page 24 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

8. Diaphragm & Condenser: the diaphragm controls the amount of light passing from

the illuminator through the bottom of the slide, there is a small lever used to achieve

the optimal lighting. The condenser is a lens system that focuses the light coming

up from the illuminator onto objects on the slide.

Why we use oil immersion lens?

• The objective lens used in microbiology is the oil immersion lens (100 x).

• As light passes through various substances (glass, air, specimens, etc.), it bends.

This bending of light is called refraction.

• The refractive index of a substance is a measurement of the extent that the

substance bends light.

• Excessive refraction can cause distortion of the image.

• At magnifications of less than 500x, the distortion is minimal. But at higher

magnifications, the distortion becomes so great that image details are lost.

General Microbiology & Immunology (PM 401) Practical note Page 13