The sun also makes our day and our night . light travels in straight lines . it comes from the sun 45 , 000 , 000 miles away 太阳还给我们创造了白天和黑夜。光是直线传播的，它来自9500万英里远的太阳。
Especially the third one describes the pinhole image formation experiment and analogies rectilinear propagation with the archery flight 这是世界上最早的针孔成像实验，也是对光的直线传播特性的最早认识。
Such waves radiate at the speed of light and are transmitted not only by line of sight but also by deflection from the ionosphere 这种电磁波以光速传播，其传播方式不仅通过无线电直线传播，还可通过电离层的反射传播。
The sound speed distribution of ocean is not uniform , so the sound ray is often curving because of refraction when sound transmitting through the sea channel . this means the time delay between the transmitter and the receiver is larger than the time delay between them in the condition that the sound ray is straightaway , then positioning error is caused 海洋中声速分布是不均匀的，因此由于折射效应，声音在海洋信道传播过程中其声线是弯曲的，声线弯曲意味着声信号从发射点到接收点的传播时延大于声信号直线传播的时延，致使定位产生误差。
