To determine if a semiconductor has a direct or indirect bandgap without prior knowledge of its bandgap energy, one can bombard the material with light and analyze the emitted wavelengths. If the emitted wavelengths differ from the incident light, it indicates an indirect bandgap, as energy is converted to phonons during emission. Direct bandgap semiconductors, like GaAs, allow for photon absorption and emission without phonon involvement, while indirect bandgap semiconductors, such as Si, primarily release energy as phonons. The ability to emit light is a key characteristic of direct bandgap materials, making them "optically active," whereas indirect bandgap materials are considered "optically inactive." Understanding these distinctions is crucial for applications in optoelectronics.