X-radiography, like infrared reflectography, is used to examine material under the top layer of paint. This is comparable to the medical use of X-rays to examine the inside of the body. X-rays are a form of electromagnetic radiation with a shorter wavelength than visible light. They therefore penetrate low-density materials relatively easily and are blocked (that is, absorbed) by higher-density materials. For instance, pigments containing heavy metals absorb more X-radiation than other pigments. These differences are visible in X-ray images: thick dots or strokes of paint containing heavy metals appear lighter in colour, or even pure white. X-radiography can be used to detect changes made during the painting process, such as overpaintings.