Cheeky Chinchillas - Logical Puzzle

Three chinchills apply for a job at the Cheeky Chinchillas School Of Philosophy.
Dean Whisper can't decide who should get the job, so she sets them a puzzle.
The first chin to solve it will get the job.

This is what she says...
"I am going to place a blue or a white spot on each of your foreheads.
If you can see a white spot, raise your right hand.
As soon as you know your own colour, lower your hand".

She actually places white spots on all 3 foreheads and so they all raise their right hands.
After a while, one of the chins lowers his hand and says, "I must be white".
Whisper asks for the explanation to secure the job.

They are all facing each other in a room where there are no mirrors.
There is a logical explanation - it is not a trick.
Do you know what it is?