Answer
X-rays are invisible to the human eye because their wavelengths are shorter than visible light. Visible light has a wavelength of about 400-700 nanometers, while X-rays have a wavelength of about 0.01-10 nanometers. This means that X-rays are about 1,000 times shorter than visible light. The shorter the wavelength of a wave, the higher its energy. X-rays have very high energy, which is why they can penetrate the human body.