The alkali metals cesium (cs) and rubidium (rb) were discovered based on their characteristic flame colors. cesium is named after the sky and rubidium after the gem color. what colors of light do you think these metals give off when heated in a flame?

As the sky is commonly known as being blue, we can deduce that cesium heated in a flame produce a blue flame. On the other hand, the name of the rubidium sounds close to the Ruby gem, which is red. Hence we can conclude that rubidium gives a red flame when heated.