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Video transcript

- [Voiceover] We have red, green, and blue lights. Why are these shadows not red, green, and blue? We have our three lights, red, and green, and blue shining on the board. It looks white. When I put the pencil in and cast shadows, the shadows are yellow, magenta in the middle, and cyan. To understand how that happens, we can turn off the lights one at a time, so that only the red light shines on the board making it red. Now when I cast a shadow, it's a dark shadow, it's the one place on the board where there is no red light. So we can think of the shadow as subtracting red light from the board, in the position of the shadow. To understand our colored shadows, I'm going to start by turning off the green light. This leaves a red light, separated from a blue light by about eight inches. That'll be important to understand the shadows. When the red light and the blue light shine on the board they scatter off the board and you see a megenta color. When I make my shadow, I see two different shadows. The light from the red light goes in a straight line and it makes a shadow here. watch that shadow as I turn the red light on and off. So this is the shadow of the red light. And over here, this is the shadow from the blue light. The blue light is in a different position so it fills in the shadow from the red light. And the red light fills in the shadow from the blue light. That's why we have red and blue shadows in a magenta board.