Why does the red balloon emit yellow light under the yellow street lamp?

In fact, it depends on whether the lamp is very light yellow, or whether the lampshade is covered with a thick translucent object to make the light look yellow.

There is a problem to be explained here. Light is generally white, and white light is actually a collection of all colors of light. If it is light yellow, it means that the translucent object such as lampshade lets more yellow light pass through, but there are many other lights. If it is very thick yellow, it means that almost no other colors pass through except yellow.

In addition, on the balloon side, if the balloon is pure red, it means that the balloon itself will absorb other light and only reflect red light.

But there is still a very important problem. Although the three primary colors of pigment are red, yellow and blue, the three primary colors of light are red, blue and green, and yellow light is composed of red light and green light.

After the combination, it only emits yellow light and the balloon can only reflect red light, so the balloon should absorb the green light in the light and reflect the red light, so the result should still be red. But visually, because there is only yellow light around, everything nearby is yellow, so you will think that everything you see is yellow, and you may also think that the balloon has a tendency to turn yellow.

Due to the superposition principle of light and color, the light reflected by the balloon itself will not produce the phenomenon that red+yellow = orange. If you see orange, it is caused by visual interference caused by environmental shadows (that is, the principle of light primary colors and pigment primary colors), but how big this interference is depends on the environment.

On the other hand, according to different situations, it is possible that lights and balloons are not absolutely red and yellow, so there is redundant white light transmitted or reflected. At this time, the balloon reflects too much green light, which will make it yellow.

If you go through a balloon, the principle is the same as before. Light only comes from yellow, and the balloon will absorb the green in yellow, so it should pass red light. If the colors of balloons and lampshades are impure, they will also deviate from white light, but they are basically red.