Final answer:
A spectrophotometer is the equipment used to measure the amount of light transmitted through a solution, which can help determine the optical density or turbidity of the sample.
Step-by-step explanation:
A piece of equipment designed to measure the amount of light that is transmitted through a solution is called a spectrophotometer. This instrument works by splitting white light into a spectrum and allowing the selection of a specific wavelength of light to use for measurement. When the chosen filtered light passes through a sample, such as a bacterial cell suspension, it can become scattered, causing some fraction of it to not reach the detector. This difference in light scattering between the sample and a control can be used to measure the optical density or turbidity of the sample, which is an indirect measure of cell density.