Microscopes 101: PHASE CONTRAST
Imagine you are in the back yard on a summer evening, and bugs are flying around the flood light that illuminates the yard. When you look directly at the flood light, the bugs are difficult to see because you are looking directly into the bright light. So you hold up your hand to cover the light, and now you see all the bugs flying beside the light very easily because the light has illuminated the bugs and refracted towards your eyes.
That’s exactly how PHASE CONTRAST works on a microscope – an annulus ring inside the condenser creates a halo of light which passes through the specimen slide. Then inside the phase objective there is another annulus ring that blocks the cone of light before it gets to your eyes. So you are not looking directly into the bright microscope light, but you are seeing the specimens (sperm) very clearly as they have light bouncing off of them amongst a darker background. Use PHASE CONTRAST for live and unstained specimens, with light as your contrasting method. But remember once you have stained the specimen, switch to BRIGHT FIELD because now color is your contrasting method.