Refraction mnemonic
If a ray of light passes across the boundary from a material in which it travels fast into a material in which it travels slower, then the light ray will bend towards the normal.
FAST TO SLOW = TOWARDS THE NORMAL
If a ray of light passes across the boundary from a material in which it travels slowly into a material in which it travels faster, then the light ray will bend away from the normal.
SLOW TO FAST = AWAY FROM THE NORMAL
The way to recall this is to remember:
Think fast sofa.
