a surface is perpendicular to the rays their intensity is at its maximum
Related Translations:
when: adv.1.〔疑问〕什么时候，几时。 W- did you see him (这以前)你几时见他的？2.〔关系，引导定语从句〕当…的时候。 He came at a time when I least expected him. 他在我最想不到的时候来了。3.从前境况差时。brag of having known sb. when 夸耀自己是某人过去的患难之交[布衣之交]。
