A spectrophotometer is a special instrument that measures how much light a substance absorbs. Every substance will transmit (reflect) and absorb light slightly differently. Like how a fingerprint identifies each individual human, knowing exactly how much red (or green, or blue, etc.) gets absorbed allows us to identify and quantify varied materials.
A spectrophotometer is used in many areas of science including microbiology, biochemistry, forensics, physics, and medical health. You can use it to measure certain ingredients in a drug to make sure it is effective and safe for consumers. You can measure bacterial growth, or diagnose a patient based on how much uric acid is present in their urine. Even non-scientists use spectrophotometers. Wine-makers, for example, use them to determine how much malic acid (reducing sugars) a wine has in it.
Scientists can also use the spectrophotometer to see how a reaction has progressed. Let’s say you’re studying iron deficiency, a condition that affects over 2 billion people. Antioxidants, vitamins, and other agents are known to help in iron deficiency, but they don’t have a single specific chemical make-up, and so they can’t be directly measured. However, they are known to reduce ferric iron into ferrous iron, a more valuable form to the human body.