Step 1: Understand the principle of flame photometry.
In flame photometry, a sample solution is sprayed as a fine mist into a flame. The heat of the flame excites the metal atoms present in the sample, raising their electrons to higher energy levels.
Step 2: What happens when excited atoms return to ground state?
When these excited electrons fall back to their original (ground state) energy levels, they release the extra energy as light. This light is called emitted radiation. Each element emits at a characteristic wavelength, which is the basis of identification and quantification.
Step 3: Distinguish from other techniques.
Absorbed radiation is the basis of atomic absorption spectroscopy, not flame photometry. Scattered radiation is the basis of nephelometry and turbidimetry. Reflected radiation is used in reflectance spectroscopy.
Step 4: Applications in pharmacy.
Flame photometry is commonly used to measure alkali metals like sodium (Na), potassium (K), and lithium (Li) in pharmaceutical and clinical samples.
Step 5: Conclusion.
Flame photometry specifically measures the emitted radiation produced when thermally excited atoms return to ground state.
Answer: Option (3) — Emitted radiation