A: The "best" wavelength depends entirely on what you want to see. UV light is invisible, but it makes different materials glow (fluoresce).
365nm (Longwave): The industry standard. Choose this for checking currency/IDs, leak detection, forensic analysis, finding pet stains, curing UV resin, and hunting certain minerals (like Yooperlite).
254nm / 310nm (Shortwave / Midwave): Specialized tools. Choose these primarily for geology and advanced mineral identification. Many stones that do not react to 365nm will glow brightly under these wavelengths. Note: These shorter wavelengths require extra safety precautions (eye/skin protection).