Definition: X-ray: Light that is so blue humans cannot see it. A band of the spectrum between the ultraviolet and the gamma-ray. Photons of X-ray light are more energetic than photons in the ultraviolet but less energetic than photons in the gamma-ray. X-radiation can go through human skin tissue but is stopped by dense bones. This property thus makes X-rays valuable in medicine. It is also useful in observing outer space. Scientists used X-rays to discover strange auroral disturbances at Jupiter's north and south poles.