The solar spectrum is the band of colors produced when light is dispersed by a prism.Polychromatic light consists of several colors. Monochromatic light consists of one color.

White light is a combination of all the colors of the spectrum. White light is not a color, but is a combination of colors. Similarly, black is the absence of all colors.

Remember that electrons can be caused to oscillate when exposed to light. Some frequencies cause a great deal of oscillation (very resonant), while others do not.

If the electrons are very resonant at that frequency, that color is absorbed. If the electrons are not very resonant at that frequency, that color is re-emitted.

An object can only emit (or absorb) colors of light that are incident upon it. A “blue” object won’t look blue if only red light is shining on it. There is no blue to be re-emitted (reflected).

Fluorescent lights and incandescent lamps produce different combinations of frequencies. So objects will appear different under each type of light. Objects will also appear different in sunlight.

The color of a transparent object depends on the color of light it transmits. The material that absorbs the other colors is called a pigment.

White light from the sun is a combination of all visible frequencies. White light also results from a combination of red, green, and blue light.

Red and green light combined produces yellow. Red + blue = magenta (bluish red).Green + blue = cyan (greenish blue).

Red, green, and blue are the additive primary colors. Color television uses these three colors to produce all other colors.

Red + green = yellowRed + blue = magentaBlue + green = cyanWhen you add one more color, it produces white light.So, yellow + blue = white

magenta + green = whitecyan + red = white

Two colors that combine to form white light are called complementary colors.Yellow & blue, magenta & green,and cyan & red are complementary.

Any color of light has a complementary color which, when the two colors are combined, produce white light.

Mixing colored pigments does not produce white pigment. It produces a black or brown color. Pigments absorb some frequencies, and reflect others. When pigments are mixed, that increases the frequencies that are absorbed, and decreases the frequencies that are reflected.

Mixing colors of light is an additive process. Mixing colors of pigment is a subtractive process.

Magenta, yellow, and cyan are the subtractive primary colors.

Light from the sun causes the particles in the air to vibrate and reemit the frequencies that caused the particles to vibrate. The colors that cause the most vibration of air particles are violet and blue. This re-emission is called scattering. Our eyes are more sensitive to blue light than violet, so the sky appears blue.

Clouds have water droplets of many sizes, so all frequencies are scattered. Therefore clouds appear white. High up in the sky, there are too few particles, no scattering means a black sky.

At sunrise and sunset, the light goes through more atmosphere because of the angle. All the “biv” end of the spectrum is scattered out and only the “roy” remains.

Water resonates with the red frequencies of light. As you go deeper in water, most red is filtered out and the greenish-blue color of water remains. The surface blue color is reflection of the sky.

Every element has a characteristic color when it is made to give off light. These colors can be divided up into the frequencies that compose them with a spectroscope. This shows up as a series of lines called the line spectrum.

A spectrometer can detect the presence of any element since line spectra are so specific for each element.

General wave properties are easily recognized for light, sound, and water disturbances.

Electromagnetic theory predicts that light is a transverse wave, but before EM theory, light was believed to be composed of longitudinal waves.

Interference and diffraction, while they seem to prove that light is a wave, do not distinguish between longitudinal and transverse waves. What evidence is there for transverse waves?

Fresnel observed that a light beam incident on calcite was separated into two beams which did not produce interference.

Young explained this by suggesting that light was made up of transverse waves and the two images formed in the crystal had oscillating planes that were at right angles to each other.

Young called this a plane-polarization effect. Plane-polarized light is light in which the oscillations are confined to a single plane that includes the line of propagation. Transverse waves can be polarized, longitudinal waves cannot.

In unpolarized light the oscillation planes are randomly oriented about the line of propagation. We can represent unpolarized light by a system of light vectors.

Usually these vectors are resolved into one vertical and one horizontal vector.

This selected orientation is entirely arbitrary.

Polarization can occur from scattering, refraction, and selective absorption of light. A single “polarizer” will filter out light rays of one orientation and allow the other orientation to pass through.

A second polarizing filter will allow the same rays to pass through ( if oriented the same) or will screen out the rays (if it is rotated 90°). This second filter is called an analyzer.

Certain crystals absorb light of one polarization and transmit light in the other polarization plane. This property is called dichroism.

Quinine iodosulfate is one dichroic crystal. When these crystals are embedded in cellulose film they form Polaroid. It was invented in 1935 by Ed win H. Land.

Sunlight reflected from surface water, a flat highway, or a car surface is partially polarized. Polarized sunglasses (with the lenses correctly oriented) are helpful in screening out this glare.

Calcite and many other crystals show the property of double refraction. The unpolarized light divides into two beams at the crystals surface. One of the polarized beams follows Snell’s Law, the other does not.

Calcite crystals can be polished, cut, and recemented together in such a way that one polarized beam is reflected at the cemented face. This can produce a beam of completely polarized light and is called a Nicol prism.

If the two waves interfere, their oscillations (or components of these oscillations) must lie in the same plane.

If polarized light is directed on a doubly refracting crystal (like calcite) at the proper angle, the emerging beams are found to interfere as they pass through an analyzer disk.

Those waves from the two beams (which normally would not interfere because they do not lie in the same plane) having a phase difference of an odd number of half wavelengths interfere destructively, the corresponding color to that wavelength is removed, and the color’s complement is observed.

Materials that become doubly refracting when subjected to mechanical stress are said to be photoelastic. Glass, Lucite, and clear plastic forks are examples. Interference fringes show the strain patterns when photoelastic material is placed between polarizing and analyzing disks.

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