medium of varying refractive index

例句：

1. 1. The book is a great medium for learning about history.
   这本书是学习历史的良好媒介。
2. 2. The teacher was the mediator between the two parties.
   老师是双方的中介人。
3. 3.The scientist used a medium to transmit information through a vacuum tube.
   科学家使用一种介质通过真空管传输信息。