Hey Gnome Nation : Wanna see a test tube disappear? Check THIS out. Refraction Action. #Hashtag180… https://t.co/xzd1ELY8Nk

Hey Gnome Nation : Wanna see a test tube disappear? Check THIS out. Refraction Action. #sci6p12 #Hashtag180 #SalemProud http://pic.twitter.com/HvVz9QLJdu

— Bill Ferriter (@plugusin) May 23, 2017

See this video for a more detailed explanation of refraction. At 1:30, he explains that refraction only occurs when the speed light travels changes.