As dark clouds gathered above the Irish countryside, our intrepid feline traveler Henri hurried down a country lane calling out impatiently, “come along Cassie.”

“But I want to pet the sheep,” answered the kitten’s voice from the nearby field.

“Not now,” responded Henri in a frustrated tone. “We’ve got to get to Duhallow before it starts raining. You can see the sheep tomorrow.”

“Okay,” grumbled Cassie climbing over the fence and joining the older cat.

They hurried onward, but had only gone a short distance before large raindrops began pelting them.

“I’m getting wet,” mewed a distressed Cassie.

“So am I,” answered an annoyed Henri while looking around for any kind of shelter. “Follow me,” she ordered after seeing what looked like a barn.

Running through a field, the two wet cats reached what turned out to be a decrepit old barn that looked like it hadn’t been used in years. With no other choice, they made their way inside. A portion of the roof and walls had fallen in, giving them barely enough light to see. Henri looked around and checked the remaining walls.

“Should be safe enough to wait out the storm,” she concluded as she and Cassie shook themselves off and found an old bench to sit on.

“Wouldn’t be surprised if this place is haunted,” whispered Cassie excitedly.

“Don’t be ridiculous,” scoffed Henri, “there’s no such thing as ghosts.”

Both cats jumped and their fur stood on end as a voice suddenly called out, “Hold there! Who dare enters the castle of Sir Thomas the Brave?”

However, their fear quickly subsided when they saw the small orange kitten holding a small wooden sword.

“You frightened us,” scolded Henri.

The kitten looked down sheepishly, “sorry, I was just playing knight.” He lowered his sword and held out a paw. “I’m Tommy McLearen. I didn’t mean to scare you.”

“Oh, you didn’t frighten me,” declared Cassie. “After all, I’m Lady Cassiopeia, Knight of the Round Table.”

Tommy looked at Cassie and shook his head. “You can’t be a knight, you’re a girl.”

“I can too,” countered Cassie. “There’s no rule saying a girl can’t be a knight. Isn’t that right Henri?”

Henri smiled, answering with a twinkle in her eye, “not according to my friend Sir Rupert and since he’s an actual knight, he would know.”

Tommy thought it over for a moment, “very well then,” he replied solemnly. “As a fellow knight, you may join me on my quest.”

“A quest?!” exclaimed an excited Cassie. “What is it?”

Tommy sat down beside them. “Only to find the greatest treasure known to cats: the pot of fish at the end of the rainbow.”

Henri did her best to keep from laughing.

“I’ve heard of this treasure,” answered Cassie trying not to giggle, “no matter how much fish you eat, the pot is always full.”

“And it’s guarded by a dragon,” added Tommy.

“He’ll be no match for us,” declared Cassie, raising a paw. “Sir Thomas and Lady Cassiopeia shall vanquish him.”

The three of them waited for the rain to stop with Cassie and Tommy happily coming up with additional creatures guarding the magical pot of fish. By the time the rain stopped, they’d added another dragon, a Minotaur, and a three-headed dinosaur.

Leaving the barn, the three cats were greeted with the beautiful sight of a rainbow extending across the sky in front of them. The feel of the sun on Henri and Cassie’s backs felt good as they were still slightly damp.

“There’s our magical signpost to the treasure,” cried Tommy pointing at the rainbow.

“Oh, it’s not magic,” corrected Henri raising her paw, “it’s Physics!”

Cassie rolled her eyes and whispered to the confused Tommy, “don’t mind her, she’s always saying that.”

“To the treasure,” called out Cassie as she and Tommy began running towards the rainbow. Henri did her best to keep the two kittens in sight. However, as much as they ran, the rainbow never got any closer. Reaching the top of a large hill, the two kittens let out cries of frustration as the rainbow had vanished. Henri staggered up to join them.

“Dire magic has once against kept me from the fulfillment of my quest,” muttered Tommy. “Some evil sorcerer has enchanted the rainbow so I can never reach it.”

Henri considered explaining, but seeing the look of disappointment on Tommy’s face, decided to keep quiet. However, the sight of a small lake on the other side of the hill gave her an idea.

“But, Sir Thomas,” she declared pointing at the lake, “you have indeed completed your quest. The treasure lies before you.”

The two kittens stared at the lake and then at each other. “Of course,” they cried out together. “A pot of fish which never runs out.”

“Clearly, a magician has cast a spell to disguise it,” suggested Cassie.

Tommy nodded, “very clever. No wonder I could never find it.”

Henri smiled. “Well, I think two brave knights deserve a reward for all their hard work. I’ll bet that village by the lake has a restaurant where we can feast upon the treasure.”

Cassie clapped her paws together, “a fine reward indeed.”

Tommy considered the invitation for a moment. “I’d better check with my parents to see if it’s all right.”

“Well then, lead on Sir Thomas and we’ll see if the King and Queen will give you leave to join us,” chuckled Henri.

Tommy’s parents greatly enjoyed meeting our two feline travelers and gave their permission for him to join Henri and Cassie for dinner. Henri and the two knights were soon enjoying a delicious meal together as Tommy and Cassie took turns regaling each other with tales of their knightly exploits.

I wonder if Tommy was right about the rainbow being enchanted so they could never reach it? Henri clearly didn’t think so. Even so, it did seem strange that the rainbow kept moving away from them. After all, what is considered magic is often unexplained Physics. Let’s examine how a rainbow forms to see if that’s the case here.

We start with something that’s probably surrounding you at this very moment: light. Probably not high on your list of exciting things, but there’s more to light than meets the eye (sorry). The ancient Greeks (like that Aristotle guy) believed light to be pure white with color being caused by impure Earthly matter. However, Isaac Newton (yes, him again), in his 1704 book Opticks, proved that white light itself is actually a combination of different colors.

Newton directed sunlight through a prism producing seven colors: red, orange, yellow, green, blue, indigo and violet (these seem familiar). Placing an upside down prism after the first caused the colored light to combine back into white light. Now if Aristotle was right, the second prism (since it’s impure Earthly matter) should have added more colors, not make it white again. Nice try Aristotle, thanks for playing.

Hold on there, if white is the absence of color, how do we get white light by mixing a bunch of colors together? Try that on a sheet of paper and all you get is a mess. Well light is a bit more complex than crayons.

Light is (cue dramatic music) electromagnetic radiation. X-rays, radio waves, and even the Wi-Fi connecting your phone to the internet are other types of electromagnetic radiation. Each one of these is made up of energy waves which oscillate (go back and forth) a different number of times each second. X-rays oscillate 3x10e16 times per second while depending on its color, the light we see oscillates anywhere from 4x10e14 to 8x10e14 (8 followed by 14 zeros) times per second. The number of oscillations per second is called frequency.

Our eyes use this frequency to tell us what color we’re seeing. So for a nice red apple, our retina senses waves with the frequency of red light and through the optic nerve tells the brain the apple is red.

That’s all well and good, but what does this have to do with different colors of light combining to make white light? Different colors of light have different frequencies and if you keep adding frequencies, we’ll eventually have a solid band of energy instead of the oscillation for one color alone. This confuses our eyes which seeing no oscillations tells the brain there’s no color and we see white.

Look around where you are for something that has color, a wall, a cat, or even a piece of clothing. Did you know that none of those objects actually have color? Each of them absorbs certain frequencies of light and reflects others. So a blue wall would absorb every frequency of light except for the one corresponding to blue. A white sheet of paper reflects all light frequencies equally.

I suppose this all seems rather hard to believe, but it’s not hard to prove. In the summer have you ever noticed that the sidewalk is cooler than the street? That’s because the black asphalt absorbs all the light energy (ouch that’s hot) while the lighter colored sidewalk reflects some of the light energy.

Yes, there’s a lot more to light and color that most of us realize. Now what does all this have to do with rainbows? Quite a bit. One interesting characteristic of light (and all electromagnetic radiation for that matter) is that when it moves from one material to another (like from air to glass) it’s direction changes. This is called refraction.

Ever sit by a pool with your legs in the water or put a straw into a glass of water? Your legs and the straw appeared to bend, didn’t they? That’s due to the refraction of light.

So what causes refraction? The thing about light is that it’s always in a hurry. The speed of light in a dense material (like glass or water) is actually slightly less than in air so light takes a shortcut. It changes its direction so it gets through the denser material faster to make up for the slower speed. This is called Fermat’s principle of least time and was developed by the French mathematician Pierre de Fermat. (Henri suggested I include the equations behind refraction (Snell’s Law), but I distracted her with a cat treat)

Refraction’s a neat trick, but where do the colors come from? It turns out the speed of light in a material like water or glass changes based on the frequency of the light. That means the each color of light takes a slightly different path. That’s how a prism works.

So there you have it, the refraction of the sunlight through the water droplets in the air after it rains produces a rainbow. Imagine walking outside after a rain and breathing in the clean, cool air as you hear the wet ground squish beneath your feet. Looking up, you see it, a glorious rainbow. Shading your eyes from the sun you gaze up …

Wait a minute. Something’s not right here. I don’t know about you, but I’ve never had to shade my eyes from the sun when looking at a rainbow. Also in today’s story (which according to Henri is accurate), the sun was shining on the cats’ backs when they looked at the rainbow. Clearly we’re missing something here.

Ever sit by a pool or lake and have to wear sunglasses because of the glare coming off the water? Yes, water not only refracts light, but it reflects it too. So a rainbow is actually the reflection of sunlight being refracted by water droplets in the atmosphere. The following picture shows what happens to sunlight entering a single water droplet.

Notice anything about the order of the colors exiting the droplet? That’s right, they’re opposite of what we see in a rainbow (yes, the picture’s correct). So why do we see red at the top of a rainbow? Look at the angle of the different colors leaving the droplet. These angles are due to the different frequencies of the colors and how they reflect off the back of the droplet. The red lights always exits the droplet at a 42 degree angle compared to the incoming sunlight while the blue light always has an angle of 40 degrees.

So we’re out looking at a rainbow. Look what happens to the light from the droplets. For droplets higher in the sky the red light reaches us while the blue light shoots above our heads. For lower droplets, the red light crashes into the ground before reaching us while we see the blue light just fine.

Boy, quite a bit goes into forming a rainbow, starting with the very nature of light itself. Considering all that’s involved, one could say it’s magical. And speaking of magic, was Tommy right about “dire magic” causing the rainbow to keep moving away from him? Sorry young kitten, but it’s for the same reason we see red at the top of the rainbow. Back in our story, the two kittens took off after the rainbow while poor Henri tried her best to keep up (don’t call her fat). Here’s how each of them saw the red part of the rainbow.

Because the angle of the light of the rainbow relative to the incoming sunlight must always remain the same, it will always appear to be the same distance away.

So there you have it. Not only is a rainbow beautiful to look at, but it also provides a wonderful example of the splendor of physics. Just look at everything that has to work together to decorate the sky with that glorious arc of colors.

Some would argue that looking at the science behind something like a rainbow somehow diminishes its grandeur. Nothing could be further from the truth. If anything, understanding rainbow formation makes it that more wondrous and enhances our appreciation of it. I’m quite certain Henri would agree.