Ria says that she can make the writing on a yellow card disappear by shining a green light on it. What color pen must she write in? Explain.

Ria must write on the card with a red pen. When the red letters are lit with a green light, they reflect a color that our brain perceives as the yellow color similar to the color of the card.

Based on the biological response in the cells of the human eye (cones), in the presence of certain frequencies of light and its interference, there are three colors that can be called primary colors; these colors are red, green and blue. These three colors are considered as a primary color set of light, since one can obtain a wide range of visible colors with them.

When we mix two of the ideal primary colors, we get the following results:

Blue + Green = Cyan

Blue + Red = Magenta

Red + Green = Yellow

So, when the green color is added, the red writing will be reflected in the color yellow and no one will be able to distinguish the writing on the yellow card.

When the three primary colors are mixed, the human eye perceives the white color.

Blue + Red + Green = White