在几何学中，镜像，顾名思义，就是物体相对于某镜面所成的像。

在二维空间里，一个物体（或二维图形）的镜像就是该物体在某平面镜中反射出来的虚像。这时镜像与原物有同样大小，但不尽相同，除非原物本来就是反射对称的。

Specular reflection is the mirror-like reflection of light (or of other kinds of wave) from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Such behavior is described by the law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection ( in the figure), and that the incident, normal, and reflected directions are coplanar. This behavior was first discovered through careful observation and measurement by Hero of Alexandria (AD c. 10–70).