UV Flashlight

This flashlight emits light at ultraviolet wavelengths, which are higher energy than visible wavelengths and are not detectable by the human eye. Obviously, this means that you can't see the light. If you do actually see the light emitted by this flashlight, they you have SUPERHUMAN VISION and have a future fighting crime*!

*Note: this is not actually true. The light is mostly ultraviolet, but also emits a little in the violet and blue wavelengths.