X光是一種具有輻射性的電磁波，有特殊的穿透能力，能夠穿透身體內的不同組織，從而產生影像，並投射和記錄在特製的感應器及菲林片上。雖然X光具有輻射性，但是輻射的劑量是非常低的：照一次肺片所承受的輻射劑量，大概只等於乘搭一程，由香港往日本的飛機航班時，所接受之宇宙輻射量。

X-Ray is an electromagnetic wave. It can penetrate different tissues in the body to produce images, which are recorded on special films. The radiation dose is very low. The radiation dose received in a Chest X-Ray is roughly equal to the cosmic radiation received during a flight from Hong Kong to Japan.