Bright beams of sunshine shone through the high windows of Twilight’s castle and scattered off the crystal walls creating a sparkling pattern of light and shadows on the floor of the throne room. It was the hottest day of the summer and the weather had sapped everypony's energy. The residents of Ponyville were all either lying on the ground outside, enjoying the warmth of the solar radiation, or had retreated to cooler spots indoors.
Rainbow Dash lay on her back with her head propped against the arm of her throne staring at a page of the Wonderbolt Advanced Aerial Manoeuvres Manual through her sunglasses, while sucking at a mango and passionfruit smoothie through a straw. Occasionally she flicked her tail up between her legs to shoo away a passing fly. Opposite her, Rarity sat in a more elegant poise, flipping through the latest edition of Cosmare.
Twilight Sparkle stood to one side, with a gleeful smile, delivering a lecture to Apple Bloom, Sweetie Belle and Scootaloo. Not a princess who would let a thing like an excessive heatwave interfere with her Twilight Time duties, she had spent the previous evening meticulously planning a science lesson. The crusaders listened to this with the angelic expressions of good little fillies, hoping that once this was done they would get an ice cream.
“A natural rainbow is formed when sunlight is reflected by rain droplets. The light passes through the droplet and is reflected at the back. As it passes through the water it is refracted – bent by an angle. But the angle is different for different colours, so the white sunlight is split into the full spectrum of colours.”
To illustrate this she used her magic to pull the curtains across the window behind her, leaving a small gap through which the light could shine. She then levitated a small glass crystal into the sunbeam, so it split the light and projected an array of colours on the wall.
“Wow!” said the three crusaders together. The image of the rainbowfied crystal reflected in their eyes. Rainbow Dash raised her sunglasses, unable to see her book in the darkened room. She turned to watch Twilight and the fillies.
“Where do all those colours come from?” asked Apple Bloom.
“They’re all part of sunlight,” replied Twilight. “White light is a mixture of all the colours in the rainbow.”
“But why does it split them up like that?” asked Scootaloo. “Why is red different from green and blue?”
“They have a different wavelength,” said Twilight. “Light is a wave which can ripple across space like a water wave can ripple across the surface of a pond. But the red waves are longer than the blue. When the waves move into a rain droplet, or a glass crystal, they slow down, and the change of speed bends the whole wavefront around.”
She projected an animated illusion before the audience to illustrate this principle, using her magic to cajole the atoms in the air to temporarily forget the laws of physics and emit coloured light in the image of a set of moving sine waves.
Source
“The wavelength of light is very short – less than a millionth of a metre – violet light has a wavelength of about 400 nanometres, blue light has a slightly longer wavelength, then comes cyan, green, yellow, orange, and red light is the longest – up to 700 nanometres… And what’s more…” she added for an encore, “There are colours beyond these. Infrared light has a wavelength longer than red. While ultraviolet light has a wavelength even shorter than violet. Our eyes can’t see these colours, but we know that they exist. Ultraviolet light can be dangerous as it can give you sunburn if you spend too long in the sun.”
This detail caught Rainbow Dash’s attention. She looked up from her book. “There are more colours than we can see? Does my tail have an ultra-red part?” She jumped up from her chair, raised her tail up in the air and swished it around.”
“Infra-red,” corrected Twilight. “That depends how cool your tail is—”
“My tail is the coolest.”
“—Infrared light is basically heat. All warm objects radiate it. While something has to be, well, red-hot to emit red light, anything or anypony will glow with infrared light in this weather. Although we don’t see it, we can feel the warmth of the sun’s infrared light. In fact—” Twilight paused as an idea came to her. “We can do an experiment.” She open a chest draw on the other side of the room and levitated a thermometer over to her. Then she repeated the last demonstration with the glass crystal, but this time lifted the thermometer into place just beyond the red fringe of light. As the infrared light heated the bulb, it sent the red bar on the temperature sensor shooting up.
“So there’s an extra strip of invisible light there?” said Scootaloo.
“Exactly,” replied Twilight.
“Is there also an invisible part of a rainbow in the sky?”
Twilight paused. She had never consider this. “That’s a good question… I suppose so… Assuming the infrared light isn’t all absorbed by the raindrops…”
“Sounds like a sound hypothesis to me,” said Rainbow. “An extra band of invisible colour makes a rainbow twenty percent cooler. So it's got to be there.”
“That’s not a scientifically correct statement Rainbow Dash,” said Twilight.
“Okay,” said Rainbow. She looked the alicorn in the eye. “Let’s do it properly. Time for an experiment.”
The Princess of Friendship looked her back. “Challenge accepted.” They bumped hooves together with decisive smiles. Without turning around, Twilight magically pulled back the curtains and sunlight flooded back into the room dazzling everypony. Rainbow put her sunglasses back in place. Twilight put on a white lab coat, then walked out onto her castle balcony and looked out at the bright afternoon sky. “I want a full rainbow over there—” she pointed a hoof to the south-west “—start a heavy rain shower just outside Everfree Sector 4, and leave the sky clear to the east. You have royal approval to temporarily override the weather schedule in the cause of science. Set it up now, while I find some instruments.”
“No problem—I can use my Wonderbolt pass to get some extra reserve clouds. You got it.” Rainbow Dash shot off the balcony and zig-zagged across the sky to collect all the clouds she needed.
“What ye gonna do Twilight?” asked Apple Bloom.
Twilight walked over to a closet, opened the door, and started rummaging among her collection of scientific instruments.
“I’m going to photograph the rainbow using a film which is sensitive to infra-red light.” She levitated a large camera, mounted on a tripod, out onto the balcony, set it to the ground, then fiddled with the lens. “And I’m going to put a monochromatic filter in to block the visible light—”
“Mono—what?”
“—Monochromatic—it will only let a single colour—infrared—through. So if we see an arc in the sky, it will be an infrared rainbow.”
On seeing Twilight attending to her camera, Rarity walked up to join her. “Twilight dear—mightn’t it be better to bring in a professional photographer for this project?”
“What?” cried Twilight. She looked offended. “I’m a perfectly good photographer. You saw that album I compiled of the historic post office buildings of Trottingham and Fillydelphia?”
“Well... you do have a unique talent when it comes to cataloguing things.” said Rarity. “But if this is to be a special image, wouldn’t it be preferable to bring in a professional who could give it a little… pizzazz? Let me see who I can find.” Before Twilight could answer she trotted out of the room.
Rainbow Dash alighted on the balcony. Behind her a series of dark clouds had been moved into view delivering a rain shower onto the meadows before the Everfree forest, brightly illuminated by the late afternoon sun. In compliance with the laws of physics a double rainbow curved across the sky.
“One totally awesome, natural rainbow, one hundred percent natural ingredients, brought to you by Equestria’s newest Wonderbolt.” Rainbow Dash perched on the balcony and posed before her artwork.
Twilight angled the camera to bring the rainbow into view and adjusted the lenses to focus the image. Rainbow Dash shot back to the artwork to shape a small foreground cloud into the initials RD, then put on her sunglasses and hovered in front of the balcony. Twilight pushed a button on the camera and all ponies heard the sound of the shutter.
“Okay… Now I just need to develop this…” She took the film out of the camera and turned to walk out the room, but was interrupted as the main room swung open. All eyes turned to watch the entrance of a blue mare with a pageboy-cut mane, purple tinted sunglasses, and a zebra-striped dress.
“I—Photo Finish—have arrived.” The fashion photographer zipped into the room followed by her entourage, then paused staring at the doorway to the balcony which framed the image of a beautiful rainbow arching over Ponyville, the end touching the cliff-top towers of Canterlot, illuminated by the late afternoon sun. “Stop! That is perfect—Yes—No no no—away with that white coat—you must wear your tiara—you are the Princess of Friendship—stand before rainbow and survey your magical realm.” The small herd of silent servile ponies who accompanied the photographer whisked away Twilight’s lab coat, brushed back her mane and plastered her face with makeup while Photo Finish took one after another shot and shouted instructions in her sharp Germaneian accent. “Yes—No! Yes… Enough! I go.”
The photographer left the room, as abruptly as she had arrived, with the photographs which would appear on the cover the next day’s Cosmare. The princess raised her eyebrows and sighed before retrieving her film and lab coat.
“Okay. As I was saying, now I just need to develop this…” She trotted away to the castle dark room. Rainbow Dash and the Cutie Mark Crusaders sat down and waited impatiently for ten minutes. Eventually Twilight returned with an enthusiastic smile, levitating a print before her.
“Success! It shows a clear band—we have scientific evidence that raindrops can reflect sunlight to form a bow in the near infra-red spectral region.
Rainbow Dash and the crusaders looked at the monochrome print showing the grey background of the rainclouds. In front of this was the white semicircle of the infra-red rainbow. Dash looked at the photographic record, also showing the outline of her proud profile in white. She gave a disappointed pout.
“Couldn’t you make it in colour?”
