The Basics of Light
By Steve Jackson
Light behaves so much like sound that for many yearsÕ scientists envisioned it as a force that moved through and invisible ocean known as the Òether.Ó Like sound, light appears to have wave length and intensity, it appears to radiate through a medium like sound, transmits energy like sound, and can even be described in terms similar to sound.
While early scientists treated light as if it were a movement of force through an invisible medium (the ether) it soon became apparent that it was not. Sound moves as waves through a medium such as air or water, but it soon became apparent to scientists that light acted like both a wave and a particle. While waves of sound moving through water are easily explained by the application of Newtonian physics and NewtonÕs laws of energy, light (and other electromagnetic waves not normally detectable by humans senses) needed a more complex explanation for how it behaved, resulting in the development of Quantum Mechanics.
Light, rather than just being a force that acts like a wave, is also a particle that acts like an object of a set mass moving through space. Thus sometimes light can be described by its behavior like it was a particle in motion, and sometimes light can be described as if it were a wave. For the purposes of laymen such as you and I, this conundrum is not a big deal. For scientists, it is a major issue standing in the way of their understanding how the universe works.
Sidebar: Newtonian Physics - Newtonian Physics (or Classical Mechanics) describes the everyday physics that we most often encounter when working in the real world. For example, there are some simple rules that anyone can remember which come into play in everyday life. For example, an object at rest will remain at rest unless something interferes with it. If someone applies force to it in the form of mechanical energy it will start moving and never stop, unless it is acted upon by another force. Of course standing on a planet other forces, such as the resistance of air and the force of gravity, so if I kick a soccer ball it will come to earth and stop.
Another aspect of classical mechanics is that energy is relative. The Earth is moving around the sun, but neither the soccer ball nor I notice because we are both likewise moving around the sun, in the EarthÕs gravitational pull. Only by changing that relationship (by kicking the ball) does change occur.
When talking about light it is useful to discuss the three ways of measuring it that is usually available to the nonscientist. Light can be measured for its intensity, or the amount of energy it carries with it. It can be measured for its frequency, or the wavelength of the light that we see as its color, and we can measure the polarization, or the angle at which the light photons are in when they enter the eye (polarization is not normally detectable but is used in devices such as sky filters and sunglasses). Of the three, intensity and wavelength are the most important for photographers.
Light is one of the two senses normally used in electronic mass communication to communicate information. Light is generated by a number of objects, both natural and man made, and moves through the atmosphere relatively easily, blocked mostly by larger particulates (pollution and dust) or by water vapor.
Our perception of light comes from our eyes. The eye is a complex organ that focuses light through a lens, restricted by an iris (to control the amount of light) onto a very sensitive set of organs that turn light photons into electricity.
Sidebar: Sir Isaac Newton – Humans tend to see light as a single color because their brains adjust for subtle differences in the color of light reaching their eyes. A grocery store with florescent lights is ÒgreenishÓ (contains more green light) than sunlight at noon, but both appear ÒwhiteÓ to most people. In fact, both lighting situations appear like ÒnormalÓ light because the human brain is capable of adjusting itself to handle inputs of difference values. If we were to take a camera and take a picture in the supermarket, then take one outdoors, making sure the camera configuration did not change (many cameras can now change their response to light without the operator making a choice) then we would see the difference between the lighting environments. Sir Isaac Newton, as easy as 1666, showed that light was made up of many different colors, and their mixture appeared white to a healthy human. He did this by using a prism to split light into its individual wavelengths. Rainbows do something similar.
There are several types of light sensitive tissues in our eyes. Rods, which function at low light, are spread out around the edges of our eyes. They do not see color very well, but they easily detect even small amounts of light. At night, you can often see well by using the edges of your vision rather than the center as you would during the day. The second form of light sensitive cells in the human eye is cone-cells. Cone cells are far less sensitive to light than rod cells, but they have the advantage of coming in three distinct types, called long, medium, and short cones for the wavelength of light they are sensitive too. Sometimes people will refer to these as Red (L), Green (M) and Blue (S) cone cells, but this is technically incorrect, we think of these colors as primaries in visual acuity because the technology of color reproduction in electronic media uses these colors, but in fact the three cells types overlap in their ability to detect color.
Your color vision comes from the various combinations of cones seeing photons of light that has been reflected from a surface. If a surface reflects long wavelength light but absorbs medium and short wavelength light, then it will appear red to the eye. A surface that absorbs most types of light will appear black to the human eye. A surface that reflects most light will appear white. A surface that not only reflects light but is smooth enough to reflect it back in a cohesive order will appear silvered, possibly even mirror like if the reflection is accurate enough.
The human perception of color, and its use by humans has resulted in a branch of study called Òcolor theory.Ó Color theory looks at how color is perceived, reproduced, and recorded,
Sidebar: Color Blindness – When one or more color cones in the human eye are unable to respond to light, the person is said to be color-blind (technically retinal color-blindness because the fault is in the retina, where the cones reside). There are many forms of color blindness, the most common being red-green color blindness that affects the ability to see long wavelength light. The most common form of color blindness, Duetranomalous color blindness, affects around 5% of all men. People with this color blindness cannot see greens well because of a problem with their M Cone cells.
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Color Theory
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