说到光导纤维其实就是一个汉堡包，一共三层：
1.a core made of highly pure glass with a high refractive index for the light to travel,
2.a middle layer of glass with a lower refractive index known as the cladding which protects the core glass from scratches and other surface imperfections, and
3.an out polymer jacket to protect the fiber from damage.
另外一个比较好的光导纤维的例子就是全内反射现象（The total internal reflection phenomenon）可以enables light to travel long distances with little loss of intensity.
不知道为啥复合材料里Fiber-reinforced Composites总会被问到，注意一般要带上一句Reinforcing fibers can be made of metals, ceramics, glasses, or polymers that have been turned into graphite and known as carbon fibers.
如果你这方面准备比较充足，完全可以加上一句纤维加强型复合材料的特点，也就是纤维能加强基材的模量。
那么很大几率APS审核官就会追问你how：
教科书的标准答案就是The strong covalent bonds along the fiber’s length give them a very high modulus in this direction because to break or extend the fiber the bonds must also be broken or moved.
当然，例子一般是少不了的，such as a time-trial racing bicycle frame which consists of carbon fibers in a thermoset polymer matrix.应该就能应付的妥妥的。
另外几种形式像Particle-reinforced composites，一般就会问及用途，答案也比较简单Particles are used to increase the modulus of the matrix, to decrease the permeability of the matrix, to decrease the ductility of the matrix. 基本就能满意。
