With Jin joining in, the communication study meeting continued straight through lunchtime and into the afternoon.
"In the case of light, the issue was that it traveled in too straight a line, but..." Jin began, breaking the silence. "What would happen if we used radio waves instead?"
Since they were invisible to the naked eye, it was difficult to picture.
It was Reiko who answered him.
"Father, I can see radio waves, you know."
"Huh?"
Reiko had just casually claimed she could see them.
"I had not mentioned it before, but when the 'previous' Father reinforced my vision, he made it possible for me to detect electric and magnetic fields, as well as see electromagnetic waves..."
"...What on earth was I doing four hundred years ago?"
Jin clutched his head, but thinking carefully about it, such a function wasn't actually that outlandish.
Given that Reiko could sense magical energy, it would honestly be stranger if she couldn't detect electric or magnetic fields. That was how Jin chose to rationalize it.
Furthermore, if she could see electromagnetic waves while using Invisible, she ought to be able to scan her surroundings much like radar... or so it should work in theory.
In other words, the current Reiko effectively possessed a limited radar capability.
"What's the difference between radio waves radiated from an antenna and those radiated through magic?"
Jin was eager to ask for future reference.
"I am unfamiliar with radio waves emitted from antennas, so I cannot say for certain."
"Well, I suppose that makes sense..."
"Radio waves emitted through magic spread out in a conical shape. In that regard, they are no different from light."
"I see."
Electromagnetic waves with short wavelengths, like millimeter waves, were supposed to have high directivity, Jin recalled, dredging through the corners of his memory.
"In that case, would it be easier on the receiving end if we used radio waves with slightly longer wavelengths...?"
In Jin's memories, the TV antennas on houses in mountainous regions—from the analog era, of course—were all thrust high into the sky.
He remembered that this was because TV signals had high directivity, making reception difficult whenever a mountain cast a shadow over the signal.
On the other hand, AM radio could be received without much trouble. That was because longer-wavelength radio waves naturally diffracted around obstacles, bending into the shadowed areas behind them.
"Let's try working with that range."
Longwaves, true to their name, had exceptionally long wavelengths, which meant they required long antennas for both transmission and reception.
Their wavelengths were so massive that they were also known as "kilometer waves."
Under normal circumstances, that made them cumbersome to use, but things were different for Jin and his companions.
"All right, let's explore that angle then!" Reinhard chimed in enthusiastically, looking ready to start building a prototype on the spot.
"Sounds good to me," Saki agreed.
"We shall leave those technical matters to the experts."
"Likewise."
Berche, Vivienne, Toa, Rodrigo, and the others seemed perfectly content to remain spectators.
"If I tweak the parameters of the magi-formula here... there, that should do it."
Jin had just finished modifying a pocket magic light—a magical equivalent of a flashlight.
"With this, it should output something close to medium-class waves. Reiko, check it for me."
Jin pointed the pocket magic light toward empty space and switched it on.
"How is it?"
"Yes, it is transmitting... however, the apex angle of the cone is roughly 170 degrees."
That meant the emission was scattering across the entire frontal area, rendering any concept of directivity moot. But Jin could already guess the cause.
"Ah, right. A lens like this couldn't possibly focus it."
A lens designed to focus visible light had virtually no effect on medium-class waves.
Incidentally, ultra-long waves would pass straight through various physical objects.
On the flip side, they were notoriously susceptible to natural background noise, such as atmospheric interference.
"Should we make the lens out of a magi-material and focus it magically?"
Saki offered a suggestion.
"Jin, what about using a light-attribute magi-crystal?"
"Ah, that might actually work. Let's give it a shot."
To put it simply, the result was a resounding success.
"Kufu, it seems the focal length changes depending on the intensity of the magical energy applied to the magi-crystal," Saki said with a delighted chuckle.
"Yeah, this is great. But wouldn't it be a bit too expensive?"
Light-attribute magi-crystals did not come cheap.
"Hmm, wouldn't it be fine if we limited its use strictly to air traffic control at airfields?" Reinhard suggested, noting it would prevent the technology from spreading haphazardly out of control.
"That's one way to look at it."
If restricted strictly to major airfields across each nation, fewer than a hundred would suffice.
"...But what about the receiving end?" Elsa asked.
Indeed, the number of potential receivers would be practically limitless.
"Besides, at this rate, it'll just be a one-way transmission," Hanna added, sounding a bit disappointed.
Everyone agreed that being unable to hear replies from incoming craft was a glaring issue.
"Yeah, there are still plenty of problems left to solve."
Yet, as Jin answered, a broad smile spread across his face.
He had never imagined the day would come when he could enjoy tinkering like this alongside his comrades once again, and he was filled with sheer joy.
"Then maybe we can find a clue if we look into the conditions for sending sound waves over long distances, or methods for picking up distant sounds?" Hanna proposed, tackling the problem from another angle.
"Right... I wonder if we could make clever use of a wind-attribute barrier."
Because of their self-imposed restriction to stick to what seemed like "commonplace" technology, a clean solution remained elusive even with all of them pooling their intellects.
Even so, it made for an enjoyable, deeply fulfilling time.
After all, they weren't bound by any strict deadlines.
Proof of that was written clearly across the bright, animated faces of everyone in the Jin Family.
Of course, creating something entirely new wasn't without its growing pains.
"...Mmh..."
As the crafters groaned in deep contemplation, Mine walked over.
"Everyone, when you hit a wall, it is best to take a break," she called out gently. "I have prepared tea in the front garden."
"...Mother, thank you."
"That is truly appreciated."
Feeling the mental fatigue setting in, everyone thanked Mine and stepped out of the laboratory for a change of pace.
"Ooh."
The weather outside was wonderfully pleasant. Everyone unwound in their own way—some settling into chairs, others lounging on picnic blankets spread across the lawn.
"The tea is great, and these senbei are delicious."
"The sweetness of the bean paste is simply wonderful. It really revives a tired mind."
"Fresh juice is definitely the best!"
Watching everyone eat, drink, and chat happily, Jin felt a profound, heartfelt sense of peace wash over him.