MadSci Network: Physics

Re: What color is made of the most colors and why?

Area: Physics
Posted By: Adrian Popa, Staff Optical/Microwave Physics
Date: Tue Mar 25 18:26:13 1997


The short answer to your question is that about 300 trillion different colors (300,000,000,000,000) combine to make all the colors that our eyes can see! Explaining Why will take a bit more discussion. What color is made of has many different meanings to people who use color for different purposes.

To a scientist it is described by wavelengths of the light. 
To a television engineer it is a combination of red, green and blue light. 
To a stage lighting technician it is a combination of color filters on spot lights.
To a printer it is a combination of colored inks.
To an artist it is a combination of pigments (colors) of paint.
All of these uses of light are explained by the scientists' approach so that is the one I'll use for this discussion...

If there were no hot objects in our universe at temperatures greater than 3000 degrees F (1650 C) there would be no light for our eyes to see and everything would be totally black making our eyes useless.

When our eyes do detect light it comes from two places. Light can come directly into our eyes from a hot object such as the sun, stars, a fire or a light bulb. Or, more often, light from one of the hot sources enters our eyes after passing through a colored material such as glass or by reflecting off colored surfaces such as rocks, flowers, grass, the moon and planets.

The length of the waves of light determine what color the light is. Red light has the longest waves that we humans can see and violet light has the shortest waves that humans can see. The colors of the rainbow give us the order of the wavelengths from long to short.

(The boy's name ROY G. BLIV is a trick that science students use to remember the colors of the rainbow from the longest to shortest wave length R = red, 0 = orange, Y = yellow, G = green, B = blue, L = lavender, I = indigo, V = violet. Each of these colors has trillions of different fine shades of color.)

Hot objects give off light at all colors that our eyes can see and also at colors (wavelengths ) that human eyes can not see. Scientists can calculate the number of colors from a hot object to be about 300 trillion colors from red to violet (300,000,000,000,000). This is called the PLANCK DISTRIBUTION naned for the scientist Max Planck who first discovered PLANCK'S LAW.

Which of the colors of the rainbow are brightest in the light depends on how hot the object is.

If you have a lamp dimmer in your home you can conduct the following experiment. Put a clear light bulb in the lamp and turn the voltage (dimmer knob) up until the filament in the bulb almost glows . You will feel the bulb heat up but you can't see the glow because it gives off infrared (heat) waves which human eyes cannot see.

Next, slowly turn up the voltage and the bulb will glow RED HOT. Now from PLANCK'S LAW we know that the bulb filament is about 3000 degrees F (1650 C).


When you turn the dimmer voltage all the way up the bulb will be WHITE HOT (about 6000 degrees F; 3,315 C). The bulb is designed so that it cannot be turned up to BLUE HOT (9000 F, 5000 C) for the parts would be so hot they would melt.

On a clear night you can see RED HOT, WHITE HOT and BLUE HOT stars in the sky. Now you know the temperatures of the surface of these stars! Measuring the temperatures of hot objects by the color of an the objects glow is called OPTICAL PYROMETRY.

When light from a hot source passes through the earth's atmosphere the short wavelengths are absorbed and reflected into space by the molecules in the air and the object appears more red here on earth. The setting sun is an example. Red glass works the same way it absorbs all colors except the red waves of light (there are trillions of different lengths of red waves). Green glass absorbs all the light waves except those that are green and so on. When the sun is over head the atmosphere is more blue looking with blue waves entering our eyes. The color of flowers, paints and so on are determined by the molecules on the surface of the object. Red flowers absorb all colors of light but reflect red waves into our eyes. The same is true for blue flowers and blue birds. Bright colors are produced by surfaces that only reflect waves that are more pure red or blue or green colors (What is pure red etc. is different for different viewers. Some people are color blind to red etc.).

Television engineers,(and artists and others) found out that the eye can be somewhat fooled into seeing most of the colors of the rainbow by using only three primary colors, red, green and blue. If you look very closely at a TV screen with a magnifying glass you will see the little dots of red, green and blue phosphor that are illuminated by an electron beam in the tube. By changing the intensity of the 3 electron beams in the tube, the brightness of the red, green and blue light dots can by controlled to generate all of the different colors that your eye can see on the TV screen. Part of why this is possible is based on the structure of the eye and on our brain's ability to process the signals from our eyes that travel along the optic nerve to the brain.

While colored light is well understood by scientists (lasers can be designed to produce exactly one of the 300 trillion colors of the rainbow), color vision is not as well understood by scientists. A few years ago I was at a meeting with two very famous scientists, Professor Richard Feynman a Nobel Prize winner in physics and Dr. Edwin Land the inventor of the Polaroid instant camera and the founder of the Polaroid Corporation. They got into a heated argument over Dr. Land's concepts of color vision versus Dr. Feynman's concepts of color vision. So if these two scientists could not agree about how color vision works, we still need more study in this area of science!

Regards, your Mad Scientist
Adrian Popa

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