Why is the sky blue? And why is water blue too?
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Zhuganhong asked two questions: why is the sky blue? And why is water blue too? Here's the odd part: scoop a glass of water out of the sea, and it's clear. To find out, follow one beam of sunlight: it leaves the Sun, crosses the air, then dives into the sea.
Sunlight looks white. But it's really a convoy: red, orange, yellow, green, blue and violet cars, all running together. Light is a wave. Red light has long waves, blue light has short ones. Picture it as: red cars have big wheels, blue cars have small ones. In the air, the road is covered in invisible specks: far smaller than any light wave, like an endless strip of speed bumps.
Big wheels roll over them barely shaking; small wheels hit a bump and bounce off to the side. Blue light gets knocked aside about five times more often than red. Someone worked this rule out more than 150 years ago. The bounced blue cars scatter all over the sky. Wherever you look up, blue cars drive into your eyes: the whole sky turns blue.
Violet wheels are even smaller, so why isn't the sky violet? Sunlight carries fewer violet cars to begin with. And our eyes aren't very good at seeing violet. Now the convoy plunges into the sea. Underwater there's a different kind of checkpoint: row after row of toll booths that only stop red cars.
Nobody is really stopping them. Red light has just the right energy to make water molecules wobble, and when they wobble, the red light is swallowed and turns into a tiny bit of heat. These checkpoints are lenient. Over a road as short as a glass of water, almost every red car gets through, so a glass of water looks clear. Make the road longer and it's a different story. After five meters of clean water, fewer than one in twenty red cars are left, while over ninety percent of the blue ones remain.
Orange, yellow and green get stopped too, one after another. The light that comes back out of the water to your eyes is almost all blue. The surface does mirror the sky, but that's only part of it. Dive underwater, where there's no sky to reflect: it's still blue all around, and that's water's own color. So these two blues come from opposite directions: the sky's blue is blue light knocked aside and spread across the sky. The sea's blue is red light eaten away, leaving the blue behind.
One more surprise: at dusk the sunlight comes in sideways and travels much farther through the air. The blue cars are all knocked away on the way. Only red and orange reach you. That's a sunset. Zhuganhong, where's the bluest water you've ever seen?
A glass of water is clear. The sea is blue.
Zhuganhong asked two questions in the comments: why is the sky blue? And why is water blue too?
Here's the odd part: scoop a glass of water out of the sea, and it's clear. To find out why, follow one beam of sunlight: it leaves the Sun, crosses the air, then dives into the sea.
Sunlight is a convoy of colored cars
Sunlight looks white, but it's really many colors packed together: red, orange, yellow, green, blue and violet.
Light is a wave. Red light has long waves; blue light has short ones. Picture a convoy: red cars have big wheels, blue cars have small ones.
The air is a road full of speed bumps
In the air, the road is covered in invisible specks, far smaller than any light wave. Big wheels roll over them barely shaking; small wheels hit a bump and bounce away.
Blue light gets knocked aside about five times more often than red.
The bounced blue cars scatter all over the sky. Wherever you look up, blue cars drive into your eyes, so the whole sky turns blue. Someone worked this rule out more than 150 years ago.
Violet wheels are even smaller, so why isn't the sky violet? Sunlight carries less violet to begin with, and our eyes aren't very good at seeing it.
Water is a row of toll booths that stop red cars
The convoy plunges into the sea. Underwater there's a different checkpoint: row after row of toll booths that only stop red cars.
Nobody is really stopping them. Red light has just the right energy to make water molecules wobble, and when they wobble, the red light is swallowed and turns into a tiny bit of heat.
These checkpoints are lenient. Over a road as short as a glass of water, almost every red car gets through, so a glass of water looks clear.
After five meters, less than 1 in 20 red cars is left
Make the road longer and it's a different story. After five meters of clean water, fewer than one in twenty red cars are left, while over ninety percent of the blue ones remain.
Orange, yellow and green get stopped too, one after another. The light that comes back out of the water to your eyes is almost all blue.
The surface does mirror the sky, but that's only part of it. Dive underwater, where there's no sky to reflect, and it's still blue all around: that's water's own color.
Two blues, from opposite directions
The same trip at dusk makes a sunset
At dusk the sunlight comes in sideways and travels much farther through the air. The blue cars are all knocked away on the way, and only red and orange reach you. That's a sunset.
Quick check
How do the sky's blue and the sea's blue differ?
Air's speed bumps bounce small-wheeled blue cars across the sky; water's toll booths stop red cars, so on a long road only blue is left.
