Why does a solar puppet (eg Lotte van Nivea) turn red under UV light?

Answer

Dear Ilse,

Nice question! The fact that our skin burns under the influence of sunlight has to do with the shorter wavelength of UV radiation (compared to visible light). The shorter the wavelength, the higher the energy content. We can use that higher energy content to allow chemical reactions to take place.

I cannot immediately find out which compounds are present in Nivea’s solar pupa, but the principle is called photochromism: a reversible color change through interaction with light (here UV light).

The UV light will cause certain chemical bonds to be broken (see figure), which changes the structure of the molecule. This will allow the molecule to absorb other colors of visible light. In the case of the solar puppet, more blue and green light will likely be absorbed, causing the puppet to reflect mostly red light toward our eyes. An example of such compounds can be found in the spiro-oxazines:

In the example above, the color formed will disappear again when the compound is heated in the absence of UV radiation (the molecule returns to its original structure), but there are also similar substances that return to their original “color” under the influence of visible light. .
The latter will probably be the case for the compounds in “Lotte”‘s skin.

Sincerely
Geert-Jan