Subtractive and additive, and how we see

Pigments - Color by Subtraction

Pigments reflect the color you see, and also it’s neighboring color (so green reflects green and yellow and blue)

but Pigments absorb all other colors and turn them into heat.

Primary colors: Red, Yellow, Blue

These shirts are reflecting red light and turning most other colors into heat.

These black clothes absorb all the light that falls on them and reflect no light. All the light is turned into heat.

These white shirts reflect all the colors of light that fall on them, absorbing no colors and turning none of the light into heat

Why does blue pigment + yellow pigment = green light? Yellow absorbs R-B-V, reflects Y-O-G

Blue absorbs R-O-Y, reflects B-G-V

White minus R minus O minus Y minus B minus V = Green

Yellow absorbs violet, red, and blue

light; yellowreflects yellow,

Orange, andGreen light

Blue absorbsYellow and Orange light

Between blue and yellow pigments, all colors are absorbed except for green

Both yellow andBlue pigments reflect Green light

Light - color by Addition

• No pigments involved! Only colored light!

• Primary Colors: Red, Green, Blue

• RR + GG = YELLOWYELLOW

• RR + BB = MAGENTAMAGENTA

• GG + BB = CYANCYAN

• R + B + G = WHITE

Complimentary colors - add two and get WHITE

• GREEN GREEN + MAGENTA MAGENTA = G G + B B + R R = WHITE

• BLUE BLUE + YELLOW YELLOW = B B + G G + R R = WHITE

• RED RED + CYAN CYAN = R R + G G + B B = WHITE

RED PAPER ABSORBS BLUE LIGHT; the reflected light is red

WHITE PAPER REFLECTS ALL COLORS OF LIGHT; the splash looks magenta

MagentaMagenta

What is the color of water?

Water scatters whatever light it gets

Water preferentially absorbs red, turning it to heat and leaving green and blue to scatter back to the eye. That makes shallow water a cyan color. WHITE - RED = CYAN

If the light continues into deeper water, the green is also absorbed, leaving only blue to scatter. WHITE - RED - GREEN = BLUE

Finally, in the deepest water, even the blue is absorbed, leaving nothing to scatter (darkness) WHITE - RED - GREEN - BLUE = BLACK

Why is the sky blue?

Air transmits R-O-Y-G, but scatters blue and violet

The air molecules are about the same size as violet and blue wavelengths (very small). So, instead of traveling straight through, these colors take a “random walk” through the atmosphere; wherever you look, you see blue light coming at you. (Violet too, but we aren’t sensitive to it.)

Why are clouds white?

After all, they are made of water

And the color of water is cyan

Answer: cloud droplets come in all sizes - from violet-light wavelength-sized drops (that scatter violet light) to red-light-wavelength-sized drops. Because all colors of light are scattered, you see all the colors at once coming from the cloud - hence, a white cloud.

Try this at home:

Take a hard, colored candy, like Jolly Rogers

Smash it with a hammer to make all different sized bits

What colors are scattered from the bits?

All of them - that’s why it looks white.

Why is the setting sun red in an orange sky?

Same reason the sky is blue, but . . .

At sunset, the light travels through a layer of atmosphere that is

Thicker and

Full of dust and pollution

So the larger air particles scatter green and even yellow light along with the violet and blue, leaving a red & orange sky and sun.

What color are we most sensitive to?

Hint: it’s the color of fire trucks (new ones) and the signs at the airport and new safety T-shirts

And, we’re most sensitive to it because it’s the color the sun puts out most intensely

What is the green flash?

Green Flash Explained

Sunlight refracts as it goes from the cold air near space through the warm air near the earth’s surface

Which way does it bend?

Blue refracts the most, just as it does in a prism. Red refracts the least.

This means that there is a red sun, and a yellow sun above it, and so forth, ending in a violet sun

Cold Air (dense)

Warm air (rare)

Earth

Cold Air

Warm air

Earth

The shortest wavelength, blue, refracts the most; red refracts the least. The air disperses (separates) the colors of white light.

Cold Air

Warm air

Earth

The image of the sun separates into many images of rainbow colors, one above the other

One flash, two flash, Green flash, blue flash!

What’s with the funny shape?

To understand this, we need to understand mirages . . . .

Mirages are an effect of refraction

When the surface air is hot and the air above is cool, light bends upward

We see the light from a cloud coming at us from the ground

It looks like the sky is on the ground

hot

cold

hot

cold

hot

cold

hot

cold

The same thing happens with the setting sun

But, the light is dispersed into different colors, and

the various images of the sun collide on top of one another

So you get a funny shape with different colors

Green sun

Yellow sun

Orange sun

Red sun

You’ve seen rainbows . . . .

Check out this!