When a light beam strikes a glass of water, a raindrop or a prism, it is slowed 
(a light beam; strikes; a glass of water, a raindrop or a prism) (it; is slowed; T:When a light beam strikes a glass of water, a raindrop or a prism)
a light beam it
strikes is slowed
a glass of water, a raindrop or a prism
T:When a light beam strikes a glass of water, a raindrop or a prism