Case: Pooja did an experiment with an equilateral glass prism in which she projected a narrow beam of white light source from one side of the surface of the prism and placed a screen on the other side of the prism. She observed certain patches of different colours on the screen. Later, she repeated the same experiment with a red-light source, however, she could only observe a red coloured patch on the screen. Again, she repeated the experiment with blue and green light sources, respectively, and could only see one colour patch on both the occasions.

The reason why she could not see any other colour when the red light was used as source because: