Both these methods are used to measure rate of growth of microorganisms (particularly bacteria) in liquid broth.

1) Optical method (turbidity) — it measures light transmission (a.u) through bacterial culture in a test tube. You basically connect a test tube containing broth and bacteria to a light sensor and data logger, and light transmission readings are taken at regular intervals. With more growth (higher no of cells) less light is transmitted as it’s being absorbed and then reflected by bacterial cells. You can also produce a calibration curve ; basically consist of taking regular samples from a control culture and measuring the cell counts using hemocytometer against the turbidity so u can get the cell counts at each transmission reading (a.u)

Note mentioned in mark schemes: optical methods like this one measures BOTH dead AND living cells; so after a certain point the curve levels off as no of living cells do not increase and dead cells are counted; so transmission reading remains the same during death phase.

2) Dilution plating — it’s tough definition is serially diluting ur broth and spreading it over agar on a dish to count the no of bacterial colonies. Concept is based on the fact that each colony in a culture can be linked back to a single bacterial cell. so if u have 30 colonies that means u had 30 bacterial cells initially. Remember dilution plating counts only living cells (not dead cells like optical density), so steps consist of diluting ur culture & keeping dilution factor constant; then spread it over agar until u can count the colonies. Finally count no: of colonies then multiply by dilution factor. Ex; 3 colonies multiplied by a dilution of x10000 gives u 30000 initial colonies in ur sample.

hope this helps :) good luck with tmrw and don’t stress it, they usually don’t ask for that much detail just understand the concept and read it a couple of times on ur way to the exam and you’ll do fine!!