下句（見補充說明）的從句部分是什么語法結(jié)構(gòu)?
The reflection principles apply even when surfaces are curved as long as the curvature of the surface is smooth,that is,as long as there is a plane tangent to the surface at the point of contact.
【譯句：即使（反射）面是彎曲的（曲面）,只要面的彎曲部分是光滑的,亦即只要存在一個平面在反射點處與此曲面相切,反射定律也可適用.】
注：此句的中文意思我能理解,但不知從語法角度怎么分析?其中的那個that引導(dǎo)主語從句嗎?