当光线垂直射入镜面的时候, when light rays strike the mirror surface vertically
I just looked this up out of curiosity, and I think 垂直 may mean "perpendicularly" here, since if I remember, the angle of the light in this episode is actually horizontal, hitting the mirror that's standing upright. (It's a new word to me either way!)
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I just looked this up out of curiosity, and I think 垂直 may mean "perpendicularly" here, since if I remember, the angle of the light in this episode is actually horizontal, hitting the mirror that's standing upright. (It's a new word to me either way!)