Studying Colour and Light Is Fun!

Students making a rainbow of colours with milk, food colouring and dish detergent.

The question "why" came up and students had some ideas.  One said that it must be a chemical reaction of some sort, so we decided to come up with the correct answer.
The secret of the bursting colours is in the chemistry of that tiny drop of soap.  Like other oils, milk fat is a non-polar fat molecule and that means it doesn't dissolve in water. When soap is mixed in, however, the non-polar portion of  the molecular soap structures in solution, break up and collect the non-polar fat molecules.  Basically, it's called fat molecule gymnastics, the food colouring molecules are bumped and shoved everywhere, providing an easy way to observe all the invisible activity.
It was neat to observe the fascination when the reaction started and kept going.
The colour display was amazing!