X射线的穿透作用
X-ray penetration
穿透作用是指X射线通过物质时不被吸收的能力。X射线能穿透一般可见光所不能透过的物质。可见光因其波长较长，光子其有的能量很小，当射到物体上时，一部分被反射，大部分为物质所吸收，不能透过物体；而X射线则不然，因其波长短，能量大，照在物质上时，仅一部分被物质所吸收，大部分经由原子间隙而透过，表现出很强的穿透能力。X射线穿透物质的能力与X射线光子的能量有关，X射线的波长越短，光子的能量越大，穿透力越强。X射线的穿透力也与物质密度有关，密度大的物质，对X射线的吸收多，透过少；密度小者，吸收少，透过多。利用差别吸收这种性质可以把密度不同的骨骼、肌肉、脂肪等软组织区分开来。这正是X射线透视和摄影的物理基础。
Penetration is the ability of X-rays to pass through a substance without being absorbed. X-rays can penetrate substances that cannot be penetrated by ordinary visible light. Due to the long wavelength of visible light, the energy of photons is very small. When it is emitted onto an object, part of it is reflected, and most of it is absorbed by the material and can not pass through the object. However, X-ray is not. Because of its long wave length and large energy, only a part of it is absorbed by the material when it shines on the material, and most of it passes through the atomic gap, showing a strong penetrability. The ability of X-ray penetrating matter is related to the energy of X-ray photons. The shorter the wavelength of X-ray is, the greater the photon energy is and the stronger the penetrating power is. The penetrability of X-ray is also related to the density of the substance. The substance with higher density will absorb more X-ray and transmit less; the material with lower density will absorb less and pass more. Different density of soft tissue such as bone, muscle and fat can be distinguished by differential absorption. This is the physical basis of X-ray fluoroscopy and photography.