In topic 2.5 you sent one colour through a prism and watched it bend towards the base by the angle of deviation. Here we send in white light — and that one change is what splits the beam into colours.
1. Present the physical scene
Picture this. It is the morning after a night of monsoon rain. The sun is out, and a fine spray of water still hangs in the air near a wall or a pump. You look up and there it is — a rainbow, a band of colours arching across the sky: red on the outside, then orange, yellow, green, blue, and violet on the inside.
Now picture something smaller that you can actually hold. A clear glass prism — a solid triangular block of glass. You shine a thin beam of plain white sunlight through a slit onto one slanting face. The light goes into the glass, travels through, and comes out of the other face. And on the wall behind, you do not see a single white spot. You see a small rainbow — a stretched-out band of colours.
Stop scrolling. Try it in your head before reading on. The light that went in was plain white, one beam. What came out was a band of many colours. Where did those colours come from — did the prism make them, or were they hiding in the white light all along?
(Answer: they were hiding in the white light all along. The prism did not paint the light. White light is already a mixture of colours; the prism just spread them out so you can see them separately. That is the whole secret of this topic.)
You can now name the scene we are about to explain: plain white light goes into a prism, and a band of colours comes out.