莫尔条纹(Moirefringes)是两组线簇重叠在一起,在其交点位置处形成的条纹.由于莫尔条纹反映了两组线簇的空间频率之差(即拍频),因此它具有放大作用,故可用来测量一些诸如角度、位相等微小变化量.本实验是利用两块空间频率相同的黑白光栅产生的莫尔条纹来测量长焦距透镜的焦距.先从理论上推导出在平行光场中,置于两块栅线呈一夹角的光栅之间某位置处的透镜产生的莫尔条纹为一组平行的直线系,它的斜率与透镜焦距之间定量关系.然后在实验中用CCD接收莫尔条纹的像,由计算机图象的斜率计算出待测透镜的焦距.

The Moire fringes are generated by overlapping of two gratings. Because the Moire fringes reflect the defference of two gratings' frequencies, they have the function of amplification of phase and angles. In this experiment, we use two gratings with the same frequency to generate Moire fringes and use them to measure the long focus length of lens. The theoretical formula is deduced. When a lens is placed between two gratings, its Moire fringes are a group of parallel lines, and the relation of the slope of the Moire fringes with the long focus length of the lens is certainty. We use CCD to get the images of the Moire fringes, and figure out the focus length of the lens with them.