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Basic Theory of Light

Intro:
This page is a bare bones explanation of what light and color are and how they work. This is a very much simplified explanation of a complex phenomena that is still yielding new secrets to this day. That means my statements are not technically 100% accurate. They are idealized, to get the concepts across without all the detail.

What is light?

Energy:

The physical cosmos is roughly divided into two kinds of stuff. One kind is called 'Matter', the other 'Energy'. Matter has mass (which we in the gravity field of Earth are used to thinking of as 'weight'). Energy has no mass. Usually these two forms are not convertible from one to the other, but they sure do affect each other! At a high enough temperature and pressure (such as inside the sun or a hydrogen bomb), matter will convert to prodigious amounts of energy, in accord with Einstein's famous "E=MC2".

Electro-magnetics:

What we call 'Light' is a narrow slice out of the middle of a vast range of frequencies of a dual nature energy called 'Electromagnetism' or 'Electromagnetic Energy' ('electro' for its electric field part and 'magnetic' for its magnetic field part).

This electric field is what people call 'static electricity', such as accumulates on your hair brush when you brush your hair, and causes a trickle of water from your metal bathroom sink spigot to be deflected if you hold the charged brush near it. The magnetic field is that familiar force in toy magnets which also makes electric motors move.
The fields in electromagnetic energy are in the form of 'waves' of an increasing and decreasing electric field and also an increasing and decreasing magnetic field.

These are waves kind of like waves in water, but in 3 dimensions. And they are not 'in' anything. That is, they go through the vacuum of space without having to move anything up and down.

We think of this kind of energy as 'beams' or 'rays' of electromagnetic energy pointing straight out from their source, with the electric field waving sideways (say horizontally) and the magnetic field also waving sideways, 90 degrees away (say vertically).
You with me so far?

Frequency:

Okay then, onto the question of frequency. Frequency of course, means, 'how often'.

For electromagnetic waves it means how many waves crests or wave dips go by you every second, or many times per second a wave repeats itself. Frequency starts right above zero. At zero there may be some energy but it is not changing over time, so it has no frequency. And frequency goes up to... well, to way farther than our minds like to go.

Most people don't realize the implications of this, but electromagnetic energy, over all of its vast range of frequencies, is the very same stuff. Even though the waves per second and the amount of energy may differ, it is all electromagnetic energy.

For example, your electric utility company sends you ultra-low frequency electromagnetic energy, which travels inside wires, at a frequency of 60-waves-per-second in the U.S.

By the way, waves-per-second is now called 'Hertz', or 'Hz', after the name of a pioneering scientist who researched such stuff.

AM radio uses a much higher frequency of electromagnetic energy. It starts around 640,000 Hz and can therefore (can you find out why?) travel through space without any wires, 'inducing' a tiny 640,000 Hz electric field and voltage on your radio's antenna. Household power and AM radio are both just waves of electromagnetic energy.

So, I bet you thought that only a very few people, with certain kinds of metal fillings in their teeth, could pick up radio or radar or such?

Actually, all of us are extremely sensitive to 'radio waves'. We depend on them constantly and use about 85% of our brain power to receive, process and understand these electromagnetic signals!

Surprised? Well, I'm fooling you a bit here, because the electromagnetic frequencies that we pick up and process and depend on so very much are extremely high frequencies. Much higher frequency (that is, the waves are much closer together) than radio, or TV, or even microwaves, like radar.

Are you beginning to see the light? The electromagnetic channels which we are tuned to are called 'light'! We sense this tiny slice (about 2%) of a vast known spectrum - and we think we know what's going on around us, never dreaming that we are blind to all but the barest slice of the vast energy spectrum. And who knows how much higher it may go?

But, unlike those who lived before us, we are now aware of and using much of this spectrum through devices like radios, cordless phones, televisions, microwave ovens, radars, lasers, telescopes and cameras.

Here is a picture of the electromagnetic spectrum we know so far

Color:

Within the narrow range of electromagnetic energy we can sense with our eyes are, of course, different frequencies. The different colors are nothing but different frequencies!
Red is the lowest visible frequency, violet the highest. Below red is the invisible 'Infrared' range (which we do sometimes sense slightly as heat). And above violet are the invisible 'Low Ultraviolet' frequencies that tan our skin and the higher damaging 'short wavelength Ultraviolet' that we try to block by putting sun screen on our skin.

Did you notice that more waves arriving per second also means more energy per second? That's why visible light does you no harm, yet short wave ultraviolet can burn your skin.

Color blends are simply mixtures of different frequencies being received at the same time, just as musical tones are mixes of different (vastly lower) frequencies being carried to our ears by vibrating air molecules banging into each other.

When we receive an equal blend of all colors, we see white. When we receive none that is black.

Color terms:
It's a lot easier to talk about color if we all use the same words to mean the same things. Therefore, there are 'official' terms used by color experts that you may also want to adopt:

Hue:
Hue is the 'color' of a color (its frequency), as opposed to its brightness or its pureness (amount of gray). An example of a hue is red.

Saturation:
Saturation is the pureness of a color. In other words, whether it is only one or a few frequencies or many. An example of saturation is a red that was 'grayed out' by mixing in some green and blue. Saturation means 'stronger' color.

Value:

Value is the brightness of a color. In other words, how it varies from black to white, regardless of its hue or its saturation. You can think of value as how dark or light a colored object would be if you took a black and white photo of it.
The word 'Tone' is sometimes used to mean a color value. For example, "That red hued text is very hard to read against that green hued background because the red and green are practically the same tone".

Names:
The official color names in the RGB additive color system used in computer graphics are:

Primaries:

Red, Green, Blue

Secondaries:

Cyan, Magenta, Yellow

I'm not sure there are official names for the Tertiaries. I call them:
Orange, Yellow-Green, Cyan-Green, Cyan-Blue, Violet, Magenta-Red.
I suppose one could make a case for calling Orange "Yellow-Red" and Violet "Magenta-Blue".

To me the most important thing in describing colors is to choose words that express information, rather than the market-driven color names that try to generate emotions. So,"blue-green" is a more descriptive name than "glade green".

Light effects:

Absorption:
Remember that we said that energy and matter interact? For example, when the electromagnetic energy called light hits some matter, usually some of it is absorbed and some bounces off as a reflection. Or, if the matter happens to be 'transparent' to light, like glass, it may just bend a bit (called 'refraction'), or go straight through without any change.

Distance:
In space, where there is nothing, light is not absorbed and goes on forever, at the 'speed of light' in a vacuum, about 186,000 miles per second (300,000,000 meters per second). The faint light from distant galaxies, which we can sense with long exposures of ultra sensitive cameras, has been traveling toward us for billions of years.

Pigments and subtraction:

Pigments subtract light energy. Lets assume you are looking at pigments (dyes or paints) under a white light.

When light interacts with matter and some is absorbed (turned into heat) there is less energy with each reflection. After enough absorption, there is no useful electromagnetic energy left.

The better a pigment is at absorbing light, the darker the pigment looks. If a pigment is 'black', it has absorbed all the light that hits it.

If a pigment is pure red, that means it has absorbed all the green and blue energy from our white light and reflected only the red.

This system of color mixing with pigments is called the 'Subtractive Color System' because colors are made by 'subtracting' other colors from white light. The 'Subtractive Primaries' are red yellow and blue. That is, you can make all other colors by mixing those three.

Light sources and addition:
In computer graphics, as in photography and television, we use a different color system. We are concerned with light sources, not light absorbers. When we work with light sources (colored lights) we add colors to get other colors. For example, we might add equal amounts of red light and blue light to get 'magenta'. So, this is called the 'Additive color system'. Its primary colors are red, green and blue. A TV tube or PC monitor is just a whole lot of little red green and blue lights. Look at one through a magnifying glass, if you want to see for yourself.

I created a demo scene in Adobe Atmosphere to show additive color mixing. It has a red, green and blue spotlight positioned at equal distances from a white screen and set to equal brightness values. Below, you can see the results.
I created a demo world to show additive color mixing.

It has a red, green and blue spotlight positioned at equal distances from a white screen and set to equal brightness values.

Below, you can see the results.
An accurate mix of additive colors, with no tweaking, using Atmosphere Radiosity.

No colors added together is black. Red and green added equally yield yellow, blue and green added equally produce cyan, blue and red added equally give magenta. All three primaries added equally (in sufficient quantity) result in white - or in lower quantities, gray.

The Color Wheel:
Colors are often thought of as degrees around a circle.
End