An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Radiation Protection

What are x-rays and how are they used in medicine?

X-rays are invisible energy waves that travel through air at the speed of light. X-ray energy is high enough that some radiation passes through objects, such as internal organs, body tissues, and clothing. In an x-ray procedure, the x-ray radiation passes through the body and onto detectors, such as film or an electronic detector linked to a computer. In general, denser objects (such as bones and calcium deposits) absorb more radiation, reducing the amount of radiation that passes through to the detector. Therefore, denser objects leave an image on the detector (that is, they appear lighter) than less dense objects (which appear darker). This is why bones appear white on x-ray images. Radiologists and other specially trained or experienced physicians can read these images to diagnose medical conditions or injuries.

Return to Frequent Questions about Radiation Protection.