BIRDS & LIGHTING - The Nature of Light

THE NATURE OF LIGHT

In order to understand the requirements and effect of light on birds, one must first understand something about what we call "light".  For our purposes, we shall begin by defining light as that narrow portion of the electromagnetic spectrum which possesses qualities that make our world, and life upon it possible.

Most people have heard of cosmic rays, and it would be difficult to find anyone who does not have experience with electricity in the form of alternating current (AC).  Cosmic rays have the ability to penetrate almost anything, as evidenced by their ability to travel completely through the earth, moving through space and matter of their own accord.  Common electricity, on the other hand, generally requires some form of conductor to guide it through to its destination.  This difference in form and behavior between cosmic rays and
electricity is what makes up the electromagnetic spectrum.  These two forms are the upper and lower ends of this distribution.

Ranging from highest to lowest then, the electromagnetic spectrum consists of:

Cosmic Rays
Gamma Rays
X Rays
Ultra-Violet Rays
Infrared Waves
Radio Waves
Electrical Waves

It is in a very narrow band of this spectrum, between the lower ranges of Ultra-Violet rays and the upper range of Infrared waves that we find what is known as visible light.  This bandwidth, and its relationship to the full electromagnetic spectrum is shown in figure 1.
 

FIGURE 1 - THE ELECTROMAGNETIC SPECTRUM OF LIGHT
 

 
It is this narrow distribution of visible light, including small components of ultraviolet and infrared that make life on earth possible.  From the ultraviolet ranges, certain metabolic processes, including the synthesis of vitamin D3 are made possible.  From the infrared ranges, temperatures conducive to the forms of life on this planet are made manifest.  The visual frequencies in between play substantial roles in the carbon dioxide cycle, and the processing of by-products of life.  The quality and quantity of this spectral range also affects the type and distribution of plant and animal life to be found in particular latitudes of the world.  No where is this differentiation more diverse than in the equatorial regions of the world, for it is here that a greater intensity and percentage of the full range of light falls, and results in a greater range of plant and animal life.  It is said that 80% of all the species of life in the world are to be found within these equatorial zones.

Although it may be difficult to visualize, all electromagnetic radiation occurs and is transmitted in the form of waves.  The full motion of a wave of light, radio, or other energy is found in the complete up and down 'cycle' of the wave.  Figure 2 gives an example of this in graphic form.  The distance between point A and point B is one full cycle.  Measurement of this motion is accomplished in one of two fashions. In general, consideration is given in the length of the wave from point A to B.  This is the wavelength, and is measured in meters, and divisions thereof.  The other method is frequency, the or how many complete
cycles are made per second.  This measure is generally used only for radio wave emissions.  For the purposes of lighting consideration, the actual length of the wave will be used.

FIGURE 2 - WAVE MECHANICS

For the spectral range which we are concerned, two different measures of wavelength are used.  The more common unit is the Angstrom.  This unit is named in honor of Swedish astronomer and physicist Anders Jonas Angstrom, who in 1862 discovered hydrogen in the atmosphere of the sun.  The other unit used in scientific measurement is the nanometer, which is .000001, or one one-hundred thousandth of a meter.  To convert between the two, simply add a zero to the nanometer measure, or drop the fourth digit (to the right) from the angstrom measure.  Examples of this are below:

4000 Angstroms = 400 nanometers
550 nanometers = 5500 Angstroms

These measures are used to show the ranges of a particular color or spectral range.

There is another measure of light, and this is used solely to mark the color balance of an natural or artificial source.  This measure is termed "color temperature", and is based on the actual temperature of the filament or gaseous emission of a device.  For a full discussion of this measure, and what it means to lighting, see the Incandescent Lighting and Black Body Radiation pages.

Light behaves in several distinct manners.  For clarity (why reinvent the wheel?) I defer to the explanations given by Encyclopedia Brittanica:

Scattering.

 When light strikes fine particles or an irregular surface, it is deflected in
 all directions and is said to be scattered. When the scattering particles are
 very small compared to the wavelength of light, the intensity of the
 scattered light is related to that of the incident light by the inverse fourth
 power of the wavelength (Rayleigh scattering). As a result, light at the
 blue end of the spectrum is scattered much more intensely than that at the
 red end.

The light from the Sun is scattered by dust particles and clusters of gas
molecules, and the scattered blue rays seen against the dark background
of outer space cause the sky to appear blue. At sunrise and sunset, when
sunlight travels the farthest, almost all of the blue rays are scattered and
the light that reaches the Earth directly is seen as predominantly red or
orange. Scattering also causes that epitome of rare occurrences, the blue
Moon (seen when forest fires produce clouds composed of small
droplets of organic compounds). Most blue and green bird feathers
involve scattering, as do many animal and some vegetable blues.
scattering also produces the blue colour of eyes, particularly the intense
blue eyes of most infants, whose yellow-to-dark-brown pigments such as
melanin have not yet all been formed so that only blue is seen against the
dark interior of the eye.

If the size of the scattering particles approaches the wavelength of light
or exceeds it, the complex Mie scattering theory applies and explains
colours other than blue; white is scattered at the largest sizes, as in fog
and clouds.

Interference.

Two light waves of the same wavelength can interact under appropriate
circumstances so as to reinforce each other if they are in phase or to
cancel each other if they are out of phase. If a beam of light falls on a
thin film, such as an oil slick on a puddle of water, part of the beam is
reflected from the front of the oil film and part from the back. Depending
on the thickness of the film, the two reflected beams can reinforce or
cancel.

When monochromatic light falls on a film of tapering thickness, a series
of dark and light bands, known as interference fringes, is produced.
With white light the sequence of overlapping light and dark bands from
the spectral colours leads to Newton's colours. The film appears black or
gray where it is thinnest and the light waves cancel; as it becomes
progressively thicker, it appears white, then yellow, orange, red, violet,
blue, green, yellow, orange-red, violet, and so on. Newton's colours can
also be seen in cracks in glass or in crystals, in a soap bubble, and in
antireflection coatings such as on camera lenses.

A large number of structural colorations in biologic systems also derive
from thin film interference. These structures usually feature multiple layers
and are frequently backed by a dark layer of melanin, which intensifies
the colour by absorbing the nonreflected light. Such colorations are
usually iridescent; the colours appear metallic and change with
orientation. Examples include pearl and mother-of-pearl, the transparent
wings of houseflies and dragonflies, the scales on beetles and butterflies,
and the feathers of hummingbirds and peacocks. The eyes of many
nocturnal animals contain multilayer structures that improve night vision
and can produce iridescent reflections in the dark.

Diffraction.

Interference is also involved in diffraction, another phenomenon that
produces colour. Diffraction is the term used to describe the spreading of
light at the edges of an obstacle and the subsequent interference that
occurs. When a monochromatic beam of light falls on a single edge, a
sequence of light and dark bands is produced; and with white light a
sequence of colours much like the Newton colour sequence appears.
("Colour: Physical and chemical causes of colour: GEOMETRICAL AND
PHYSICAL OPTICS" Britannica Online.)

The full electromagnetic spectrum is shown below in Figure 3:
 

FIGURE 3 - THE ELECTROMAGNETIC SPECTRUM

 

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Last Updated 01/07/99