The Nature of Light - mini notes

Dual Nature of Light Light shows both wave and particle behavior, a rather significant mystery. If we shine a flashlight at a wall, we see a circle of light there - no surprise. The light is behaving like particles "spaying" out of the flashlight and striking the wall in a predictable fashion. Now, shine the light through a small circular aperture, say 2 cm, cut in a piece of bristleboard and look at the wall - a small circle of light there - still no surprise. However, if the aperture is very tiny, we don't see just a small, circle of light on the wall. Instead, we see a series of concentric light and dark circles; the light energy is now acting like waves and the light and dark circles are the result of constructive and destructive interference. The effect is pronounced if we use a very bright light source and a pair of tiny, closely spaced parallel slits; the wave pattern we see now is a series of parallel bright and dark lines.

To get an idea how magical this is, imagine you aimed a garden hose at the side of your house but put a big square of wood in the way. In the wood, you had cut a pair of tiny slits. When someone looked behind the barrier to see how much water was hitting the house, they would see there what looked like a tiny ocean with ranks of crests and troughs! So, light might be described as separate particles, called photons, moving in groups or waves.

Light Waves Light is a form of energy. It is a tiny part of electromagnetic energy; other examples are gamma rays, X-rays, light waves and radio waves. Light requires no transmitting medium. Its vacuum speed is 3 * 10⁸ m/s, a value suggested by the Danish investigator Roemer in 1676 and refined by the American researcher Michelson in 1926.

Why We See Things Eighty % of what we know about our environment comes to us as light. We see objects that are luminous (emit light) or illuminated (reflect light). The latter are more common. Transparent matter, e.g., smooth flat glass, transmits light rays undistorted. Translucent matter, e.g., bubbly surface glass, paper, distorts the light rays as they pass through. Opaque matter blocks light. Many surfaces absorb light while others reflect it. (A bit of irony - although the sun blazes with light enough to blind us in an instant, it's gravity makes it sufficiently dense to be opaque. In a transparent material, a light ray travels in a straight path, but because the sun is opaque, a photon of light starting at its core travels in a zig - zag path to the surface where it escapes. The journey takes about 100 000 years!)

Color The color of a light ray entering our eye depends on three related characteristics: frequency, energy and wavelength. White light is the combination of the rainbow colors - violet, indigo, blue, green, yellow, orange and red. (But, note that it is also the sum of just the three additive primaries, red, blue and indigo.) Each color has a specific frequency and energy to which our retinal chemicals are sensitive. Each color causes a different electrical signal to be sent to the brain where it is interpreted as "light".

Newton was one of the first researchers to systematically study the nature of light. His main instrument was the prism which broke a color into its components.

There are an infinite number of subtle changes in color as we look through the Solar Spectrum. Many closely related colors have not been named and many more are not even distinguishable to our unaided eye. Although a CRT screen can create 16 million colors, we can see only about 300.

The Solar Spectrum

Combining Colors Colors combine in either the additive or subtractive processes. The combining of colored lights, e.g., at a rock concert light show, is described by the Additive Color Wheels. The Subtractive Color Wheels describe the mixing of pigments, e.g., paint, looking through filters.

The Additive Color Wheels The Subtractive Color Wheels

The colors in the large portions of the circles are called primary colors. Those in between the primaries are the secondary colors. Adjacent primaries mix to form the secondary in between. Colors opposite each other, a primary and a secondary; are complimentary and form the color (the tertiary color) at the center of the wheels, either white or black.. Also, the three primaries will form the tertiary. Note: set designers, set painters and lighting directors avoid confusion by using these terms properly.

Applications of Combining Colors Additively Ooh, How Striking!

An important aspect of color TV development was the determination of how many colors needed to be seen by the eye in order to recreate all colors. Ideas from Newton indicated that only the three additive primaries were needed, and so, color CRT screens are a pattern of thousands of glowing microscopic trios of red, green and blue circles or bars.

George Land was a researcher at the Polaroid Corporation during the 1950's. He invented the chemistry of "instant photography." His work lead to the development of the self developing photographs taken with the Polaroid camera system. One aspect of his work also was determining the fewest number of colors required to create all other colors. He thought that sometimes only two color combinations would suffice but, as yet, the three standard colors ( and sometimes more) are still used in color photography.

Applications of Combining Colors Subtractively Should We Use Bleach On This?

Stained glass windows are a striking feature of large older churches. They are intended as an expression of both piety and of artistry. A new generation of craftspeople are rediscovering the impact of stained glass creations.

Dyeing cloth was a well developed art in the Americas when Europe was still young. Colors have always had an emotional appeal and in many early civilizations, certain shades were reserved for religious, social and military leaders. The scarcity of a color was a major factor in its association with status. For example, purple was favored by Roman leaders. A story tells of an emperor walking across a bed of snails on a beach. His weight cracked some of the shells and when fluids leaked out, they stained the bottom of his robe purple. The way of obtaining this new color was thereafter kept a "secret" from the general populous.

Types of Spectra

Emission Spectra: there are three examples of this class of spectrum.

a) Continuous spectrum - the complete spectra with adjacent colors blending into each other.

b) Bright line spectrum - a series of vivid narrow sharply defined color bars set on a jet black background.

c) Band spectrum - looks like bright line spectrum in which adjacent color bars look smudged.

Absorption Spectrum: this type of spectrum and it looks like a continuous spectrum with a few black bars where some colors are missing. It is the opposite of a bright line spectrum. It occurs when a continuous spectrum passes through as cool gas cloud.

Applications of Types of Spectra Did You Get the Recipe?

One of the amazing discoveries of astronomy is the incomprehensible scale of things. A basic question posed by sky watchers is the composition of the universe. Hints about its contents come to us in the form of light. When we pass the light through a prism and examine the continuous and bright line spectra, we get a glimpse of the chemicals in the stars and blazing gas clouds producing the light. Analyzing absorption spectra tells us something about the identity of the chemicals in dark dust clouds partially obscuring luminescent bodies.

One romantic point in support of the Big Bang Theory has to do with our fascination with the night sky. The theory suggests that, like everything we see there, we too originated from star dust and so naturally feel connected in some way to heavenly objects.

Eclipses There are two types of eclipses, lunar and solar.

a) Lunar eclipses: A total lunar eclipse occurs when the full moon passes through the shadow cone extending out behind the earth. A partial lunar eclipse occurs when the full moon grazes the Earth's shadow cone

b) Solar eclipses: When the new moon passes between the earth and sun, it casts a circular shadow on the earth. The shadow has two parts, a small, central 269 km wide jet black umbra at the center of a large circular penumbra often several thousands of kilometers wide. Also, the penumbra shows progressive darking as one moves from its edge (daylight) to the zone where the umbra starts (black.) Partial eclipses are seen by people living in areas in the path of the penumbra; a total eclipse is seen by those in the umbra's path.

Interference Light waves interfere with each other just as do water and sound waves. An interesting situation of light interference is thin film interference. Dip a wire circle in a soap solution and when you remove it, rainbow colors swim over its surface. Some light reflects from the front surface of the film while some enters the film and reflects from its back. When the two groups of reflected rays mingle and interfere on their way to your eye, the result is a swirl of rainbow colors. The same type of rainbow effects are seen when light reflects from thin layers of oil or gas floating on water. Some modern paints rely on thin film interference for their color. Each color of paint contains millions of tiny crystals of just the correct dimensions to create the required color. These colors are very long lasting.

Polarization A light ray is like a million long luminous ribbons all with a common axis, all moving in the same direction. Polarization is a "filtration" process in which all but one vibration ribbon is absorbed in some material.