如果您戴眼镜，您就会知道，从光学的层面理解，焦点的意思是，将光线汇聚到您眼底的一点上。
If you have glasses you know that optically focus has to do with bringing the light to a focal point in the back of your eye.
散光有折射缺陷的透镜或其它光学系统，阻碍光线汇聚于一个焦点，形成一个模糊或不完全的图象。
A refractive defect of a lens or other optical system that prevents light rays from converging at a single point resulting in a blurred or imperfect image.
菲涅耳透镜在光学中很常用，而利用相同的原理可以汇聚声波。
Fresnel lenses with concentric rings are widely used in optical applications, however a similar principle can be used to focus acoustic waves.
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