Which was the only occasion when she got to see Danny Casey in person?

In an experiment, Pooja used a equilateral triangular glass prism and projected a narrow beam of white light source from one side of the surface of the prism. She placed a screen on the other side and saw many colours appearing as patches on the screen. But when she used a red light source, she could only see a red patch on the screen. Similarly she used a blue and green light source and could only see one colour patch on both occasions. The phenomenon that she was trying to demonstrate was:

Why did Sophie wriggle when Geoff told her father that she had met Danny Casey?

In an experiment, Pooja used a equilateral triangular glass prism and projected a narrow beam of white light source from one side of the surface of the prism. She placed a screen on the other side and saw many colours appearing as patches on the screen. But when she used a red light source, she could only see a red patch on the screen. Similarly she used a blue and green light source and could only see one colour patch on both occasions. She also could relate to another natural phenomenon that we observe on a rainy humid day as the sun comes out. What could be that phenomenon?