"Light, seeking light, doth light of light beguile;
So ere you find where light in darkness lies,
Your light grows dark by
losing of your eyes."
Shakespeare's warning, in Act 1 of Love's Labour's Lost, against spending too much of your life in academic pursuits.
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Throughout human history light has been something most of mankind has
taken for granted. It is there throughout our lives, for most of us, and
(so we assume) will always be there in the familiar patterns we
experienced as we grew up. The warming sunlight of a spring morning, the sun in our
eyes as we face a brilliant sunset. Even the light from a television or movie screen,
or the small nightlight that kept the Bogey Man away.
All are familiar and comforting to us. The very existence of the human race relies upon
light. Without it, within days, the candle of life on Earth would be extinguished.
In the past, and in some cultures even today, phenomena such as solar
eclipses, unusual sunsets or the movement of the Sun in our sky, have been
cause for great fear or have held mystic implications, because they represent a break in
that familiar pattern or an indication that there are forces at work in and around our
world that are beyond our control. These events cause us to
question our place in the universe. Even among civilized peoples, where
an eclipse is an understood phenomenon, a solar eclipse is still an
occasion for excitement and awe. An amazing sunset can cause us to stare in awe,
shake our heads at the sheer beauty, and produce a humbling acknowledgment that we are not all powerful.
To gain any understanding of light itself, we need to acknowledge
that light holds two places in our culture and in our world. On one hand, it is a source of
inspiration, it prompts us to ask deep questions about who we are and what our place is in
this world. It defines our existence at a most fundamental level. Day and night, waking and sleeping,
working and resting, living and dying.
On the other hand, light is a complex scientific effect, a phenomenon that has been described by
mathematical formulas, experimental proofs, and complex interactions with various media. The laws relating
to the propagation of light are among the most unusual in the scientific world. It displays the characteristics
of both waves and particles. It exists in forms that are both visible and invisible. It can burn, illuminate,
transfer signals and communications, it can be used to transfer images, determine material properties,
distances to objects and their velocities. It is maleable, in that it is changed by the medium it impinges
upon or transmits through.
Light has also inspired poets, artists and lovers through the ages. The reflection of light on a frothy sea, the
colors of the rainbow, the warmth of the sun as we lay on a sandy shore, ... and the darkness and uncertainty
that may come at night, alone, far from home on a black and barren landscape.
Having acknowledged the many aspects of light in our lives, this page in DaVincis World will address the technical
and scientific aspects of light.
Let's start with the Merriam-Webster dictionary definition of light:
- LIGHT
Pronunciation: \līt\
Function: noun
Etymology: Middle English, from Old English lēoht; akin to Old High German lioht light, Latin luc-, lux light, lucēre to shine, Greek leukos white
Date: before 12th century
1 a: something that makes vision possible
1 b: the sensation aroused by stimulation of the visual receptors
1 c: electromagnetic radiation of any wavelength and traveling in a vacuum with a speed of about 186,281 miles (300,000 kilometers) per second ; specifically : such radiation that is visible to the human eye
2 a: daylight
2 b: dawn
3 a: source of light: as a: a celestial body b: candle c: an electric light
4archaic: sight
5 a: spiritual illumination
5 b: inner light
5 c: enlightenment
5 d: truth
6 a: public knowledge
6 b: a particular aspect or appearance presented to view
7: a particular illumination
8: something that enlightens or informs
9: a medium (as a window) through which light is admitted
10plural: a set of principles, standards, or opinions
11: a noteworthy person in a particular place or field
12: a particular expression of the eye
13 a: lighthouse , beacon
13 b: traffic light
14: the representation of light in art
15: a flame for lighting something (as a cigarette)
— in the light of
1: from the point of view of
2or in light of : in view of
One ångstrom = 10-8 cm (0.00000001
cm).
Of course, these definitions don't really tell us much of anything new. We already know these things. Only in
the past one or two hundred years have the technical characteristics of light begun to be understood. Personally,
I believe that everything we use to define light, it's wave and particle properties, the mathematical descriptions and the
characteristics that bend, reflect, polarize or change the speed of light, all leave some missing link. We can describe
light, predict how it will behave, use it in a thousand ways to support life on Earth, but, in some way, there is a piece
missing. How does something exist that has no mass, cannot be held in hand, can move from one place to another with no motive
force at unimaginable speeds, and carries energy with it wherever it goes. I cannot grasp, at a most fundamental level of
thought, what light is.
Let's move on to explore some of the
basic properties of light as we know them today, so we can better
understand how light behaves. This will give us a chance to predict how light may
behave under various circumstances and conditions, and how it can be created and shaped for our use.
This is a fire rainbow, a very rare naturally occurring atmospheric phenomena.
The picture was captured on the Idaho/Washington border. The event lasted about an hour.
So, let's establish a few properties of light upon which to build understanding. Light consists of electromagnetic energy.
This energy exists as a spectrum that extends far beyond our visual perceptions, with higher energy being associated with the
light we call X-rays and Gamma rays and lower energy associated with Microwaves and Radio waves. These "waves" (we'll clarify
this later) have widely varying wavelengths from x-rays, with wavelengths of less than a billionth of a meter to radio waves
with wavelengths of a meter or more.
Our Sun emits light in a narrow region near the middle of the spectrum we call "visible". As humankind evolved, our eyes adapted
and became sensitive receptors to the predominant light that bathes our world. "The better to see you with, my dear!".
A very unique and unusual characteristic of light is that it exhibits the properties of both waves and particles.
At x-ray and shorter wavelengths, electromagnetic radiation tends to be quite particle like in its behavior,
whereas toward the long wavelength end of the spectrum the behavior is mostly wavelike. The visible
portion occupies an intermediate position, between infrared and ultraviolet on the spectrum, and exhibits a unique
mix of ray, wave, and quantum particle properties.
Like all electromagnetic waves, light waves can interfere with each other, become directionally polarized,
and bend slightly when passing an edge. These properties allow light to be filtered by wavelength or
amplified coherently as in a laser. The properties of reflection, refraction and diffraction (among other
advanced topics) will be discussed in this tutorial.
In radiometry, light’s propagating wavefront is modeled as a ray traveling in a straight line. Lenses, windows and mirrors
redirect these rays along predictable paths. Light is absorbed, reflected, transmitted or scattered by the medium
it passes through or impinges upon, and the optical density of the materials bends and shapes the light.
Wave-Particle Duality in Brief
Until the middle of the 1800's, the generally accepted theory of light, advocated by Newton,
was that light is a stream of tiny particles.
However, in the late 1800's, the particle picture was replaced by the wave theory of light. This
was because certain phenomena associated with light, namely refraction, diffraction and interference,
could only be explained using the wave picture.
The wave theory of light was broadly adopted and accepted until it was
found to fail to explain some observed and measured phenomena, in consistent and repeatable experiments.
Three examples of phenomena that upset this model are the
photoelectric effect,
Compton scattering,
and
blackbody radiation.
These effects could only be explained by assuming that light energy propogates as a series of
independent "corpuscles," or bundles. This gave rise to a return to
particle theory of light.
As we refine our ability to examine and measure light, it becomes clear that light has properties of both a wave and a
particle. This is an astounding discovery and is certainly outside the
realm of how we normally perceive things. Billiard balls act as particles, while oceans act as waves. Photons act as both a wave and
a particle all the time (even though it's common, but basically incorrect, to say that it's "sometimes a wave and sometimes a particle"
depending upon which features are more obvious at a given time). Just one of the effects of this wave-particle duality (or
particle-wave duality) is that photons, though treated as particles, can be calculated to have frequency, wavelength, amplitude, and
other properties inherent in wave mechanics.
The photon is an elementary particle, despite the fact that it has no mass. It cannot decay on its own, although the energy of the
photon can transfer (or be created) upon interaction with other particles. Photons are electrically neutral and are one of the rare
particles that are identical to their antiparticle, the antiphoton. Photons are spin-1 particles (making them bosons), with a spin
axis that is parallel to the direction of travel (either forward or backward, depending on whether it's a "left-hand" or "right-hand"
photon). This feature is what allows for polarization of light.
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