The human eye can only see visible light, but light comes in many other "colors"—radio, infrared, ultraviolet, X-ray, and gamma-ray—that are invisible to the eye.
There is infrared light, which, while too red for humans to see, is all around us and even emitted from our bodies. Warm-blooded animals, including humans, radiate infrared light.
There is also X-ray light, which is too blue for humans to see. X-rays are another common light source that many of us have encountered at a doctor's office. They can penetrate skin and muscles, allowing doctors to look at our bones. What you might not know is that the sun also emits X-rays. Lucky for us, the Earth's atmosphere blocks X-ray light.