What property of a substance causes it to appear white or light on an X-ray image?

The property that causes a substance to appear white or light on an X-ray image is radiopacity. Radiopacity refers to the ability of a material to obstruct the passage of X-rays. Materials that are radiopaque absorb more X-ray photons and allow fewer to reach the X-ray detector or film, resulting in a lighter area on the radiographic image.

Radiopaque materials include bone and metal, which appear white due to their high atomic number and density, leading to significant absorption of X-rays. Conversely, materials that are radiolucent, such as air and soft tissues, allow X-rays to pass through more easily, resulting in darker areas on the image. Understanding the interaction between X-rays and different tissues is crucial for interpreting radiographic images accurately.