如图所示,在双缝干涉实验中,S1和S2为双缝,P是光屏上的一点,已知P点与S1和S2距离之差为2.1×10-6 m,现分别用A、B两种单色光在空气中做双缝干涉实验,问P点是亮条纹还是暗条纹?
He was________a gold cup for his excellent performance as a reward.
如图,在正方形ABCD中,AD=5,点E、F是正方形ABCD内的两点,且AE=FC=3,BE=DF=4,则EF的长为( )