Oh yes, that's a score. We are here today at the golfing underground in Wuppertal and as you can see the whole play center is designed in bright luminescent colors. But the only light color in this whole room are those dark violet light tubes at the ceiling.

How can those light tubes create all of these bright colors? That's a question we'll answer today. Have fun and enjoy! To answer this question we are again at the University of Wuppertal to ask an expert about this. Hi Nico, good to see you. Hi Niklas, nice to meet you.

So we've just been to Golfing Underground and they have those amazing wall designs with really bright and vibrant colors. But the only light source in the whole room are those really dark violet light tubes at the ceiling. How is that possible? Well, you see, photons can be converted. That is, photons with a given energy can be converted to ones with less energy and vice versa.

On the miniature golf site for example, the photons from the black light tubes are converted into ones with less energy by interacting with the molecules of the various dyes in the walls and objects. Okay, and why does the violet or even ultraviolet light cause green and blue lights?

Well, some of the energy is lost in form of heat. I have a small model animation that explains this process step by step and we can watch that. Amazing, perfect. In every molecule there are well defined energy levels in which electrons may exist.

Relevant for this effect are the electrons from the highest occupied energy level, shown here as two arrows facing in opposite directions. This configuration is called ground state. The next higher energy level is the lowest unoccupied energy level.

Every energy level contains several different vibrational states. Electrons can move between those by absorbing and emitting thermal energy. If a particle of light, a photon of suitable energy hits the molecule, it is absorbed and resides the molecule.

Hereby, an electron jumps from the highest occupied energy level to a higher vibrational state of the lowest unoccupied energy level. This configuration is called the electronically excited state. The molecule dissipates some energy in form of heat and relaxes it into the lowest vibrational state of the electronically excited state.

The molecule emits a photon from this excited state and deactivates back into the ground state. Again, it relaxes into the lowest vibrational state, emitting once more some heat. The comparison between the absorbed and the emitted photon shows that the emitted photon has less energy.

On the miniature golf sight, the invisible UV light is converted into visible light of all colors. Well, part of the energy being lost in the process, a seed, is something we all know from engines. But you mentioned that photons can also be upconverted, so that you can make higher energy photons from lower energy ones.

How is that possible? Well, there are several possibilities. Let me explain it to you. Well, the explanation can wait. First, I want to see it. Okay, then. Come over here. Okay.

Now you can see the green light becomes blue light in the solution. I see. And the blue light has more energy? Indeed. Okay, and where does this energy come from? Is the solution warm? Well, let me switch the laser pointer off and now you can try. Okay.

No, it's not warm at all. It's more like room temperature, maybe even a little bit colder. But where do the photons get their energy from? Well, there's a little trick. Okay, can you explain it to me? Of course. Come with me. Well, there are two different compounds dissolved in that solution. Consider that these two shapes represent a molecule. The red one and the blue one.

If we now irradiate the solution with a green laser light, then a photon is absorbed by the red molecule, because only the red molecule can absorb the green photons. If the red molecule now absorbs a photon, it is excited and is now in the excited state. After a collision with the blue molecule,

the energy is transferred from the red to the blue molecule and the red molecule is now in the ground state and the blue molecule is in the excited state. This process happens once more with another pair of molecules. This one and that one. Once again, the excitation of the red molecule.

This one is in the excited state. It once again transfers its energy to the blue molecule on a collision and the red molecule is in the ground state and this molecule is in the excited state. Now, these two blue molecules collide once more. The extra energy is transferred from one molecule to the other molecule.

The first molecule is now in the ground state and the second blue molecule is now in an even higher electronically excited state. From there, it can emit a blue photon and after that, in the ground state. And this we see as blue light because it doesn't only happen one time, but many times over.

What I've just explained to you here is a simplified version of the so-called triplet-triplet annihilation. Well, the explanation was super plausible and the experiment looked amazing. But are there actually any practical applications to all of this? Of course there are practical applications. It's possible to use this process to convert photons from the low-energy red or even infrared part of the solar spectrum

into higher-energy photons which then can in turn be used by solar cells to produce electricity. Thank you so much Nico. I've learned a lot today even though I was only asking about mini-golf and I bet you learned a lot too. You will as always find a lot of other videos linked below

and you should definitely check them out. Have a good time and bye-bye.