One of the things I did today in the lab, was to check the health of some cells growing in culture, and to estimate their numbers because they need to be of a certain confluency before I can use them for any experiments. When doing tissue culture experiments in the lab, I am using a phase-contrast microscope daily, in order to inspect the cells and make sure they are growing as expected.
A phase-contrast microscope converts phase shifts in light passing through the transparent cells to brightness changes in order to form an image. The phase shifts themselves are invisible, but they become visible when they are shown as brightness variations. To make these phase shifts visible, the microscope basically separates the illuminating (background) light from the specimen-scattered light (which makes up the foreground details). The cells usually appear to be glowing, which means that they are alive and healthy, as shown in the image below.
Timelapse: Checking my glial cells to see if they are healthy and if they are growing as expected.