Adjective:

1. relating to a balloon or other aircraft that flies because it weighs less than the air it displaces

lightn.光；光线；灯；打火机；领悟；浅色；天窗adj.轻的；浅色的；明亮的；轻松的；容易的；清淡的v.点燃；变亮；照亮adv.清楚地；轻便地light boat灯船

1. Hydrogen is much lighter than air.
   氢比空气轻得多。