Final answer:
A fluorometer is the instrument that absorbs light and emits it at a longer wavelength. It measures the fluorescence of compounds by detecting the light they emit after excitation at a specific wavelength, which is then emitted at a longer, less energetic wavelength.
Step-by-step explanation:
The instrument that absorbs light and emits it at a longer wavelength is known as a fluorometer. This device is specifically designed to measure fluorescence by detecting the emission spectrum of fluorescent compounds. The process involves using one wavelength to excite the sample (excitation wavelength) and then measuring the emitted light at a different, typically longer wavelength (emission wavelength). Fluorescent compounds absorb light at a certain energy level and emit light of lower energy, which corresponds to a longer wavelength. Fluorometers are capable of conducting synchronous spectroscopy, keeping a constant difference between excitation and emission wavelengths, allowing for the detection of specific fluorophores. Fig. 9.2 illustrates that fluorescent molecules emit light at a higher wavelength than that absorbed.
In contrast, instruments like nephelometers measure the scattering of light rather than fluorescence, Atomic Absorption Spectroscopy (AAS) is used to measure the absorption of light by free atoms, and Thin-Layer Chromatography (TLC) is a method for separating compounds based on their affinities to the stationary and mobile phases, not for measuring the light emission.