How come a rainbow formed on paper when it was put on the projector?

Question

My teacher put a bunch of 10-15 pieces of paper on top of each other on the projector to cover up some of it, and a rainbow-coloured line formed on the paper when the light shone on it. How come?

Answer 1

The light from the projector.

Answer 2

I'm not quite sure I understand the details, but from the sound of it what was happening was white light was diffracting around the edge of the paper.  When that happens, the different colors in white light bend different amounts, so that the colors separate and you see a rainbow.  

Google "white light diffraction" for more details and to see if what you were seeing was physically consistent with the explanation above.