How light behaves

Interference In many cases, light can be thought of as being a wave with crests and troughs. When two light waves cross through the same spot they interfere with each other-that is they add to or subtract from each other. The two crests add together to give a larger crest. This process, called constructive interference gives brighter light than either wave would have separately.

Suppose instead that whenever a crest of one wave crosses through the spot, a trough of the other wave also does. The trough reduces the height of the crest, leaving the spot dim or even dark. This process is called destructive interference. The light that filled the screen with bright and dark lines are called fringes. Bright fringes occurred where the to waves arrived crest-on-trough to give constructive interference. Dark fringes occurred where the to waves arrived crest-ontrough to give destructive interference.

Holograms are specialized interference patterns recorded on a thin film emulsion on glass or plastic, enabling your eyes to see exactly the pattern of light waves that was reflected by a three- dimensional object. Ex.

Diffraction In Young’s experiment, the light passing through each slit spread. This type of spreading is called diffraction. Diffraction of light can be a nuisance. Suppose you attempt to see a very small object by using a high- quality microscope. As you increase the magnifying power to see the object and more closely, the object’s edges begin to blur. Each edge blurs because the light passing by the edge on its way to the eye diffracts.

Dispersion Dispersion is the spreading of light into its colors. The dispersion of white light separates the colors of the full visible spectrum. A ray of white light that passes through, a prism is dispersed into the visible spectrum. Red light is refracted the least, and the purple light is refracted the most. This is because the speed of the various wavelengths in glass is different, slowest for violet light and fastest for red light.

Polarization Polarization involves the oscillations(regular variations in strength) of the electric fields that make up a light wave. The directions of the oscillations may be presented by arrows. In most of the light we see, the arrows point in many directions perpendicular to the ray’s path. Such light is unpolarized.

Chemical Effects The energy can chemically change the surface of materials absorbing it. For example, light chemically changes the molecules of silver grains on photographic film so that an image can be recorded on it. Strong light can fade colored fabrics by chemically changing their dyes. Light changes the chemistry of the eye’s retina, so that the retina process by which they make food.

Measuring Light Scientists measure wavelengths of light in a variety of metric units. One common unit is the nanometer, which equals a billionth of a meter, or 1/25,400,000 inch. The wavelengths if light in the visible spectrum range from about 400 nanometers for deep violet to about 700 nanometers for deep red. The frequency of any equals the ratio of the wave’s speed to its wavelength. Frequencies are measured in units called hertz.

The Brightness of Light Scientists use various units to measure the brightness of a light source and the amount of energy in a beam of light coming from that source. The amount of light produced by any light source is called the luminous intensity of that source. The standard unit used to measure luminous intensity is the candela. For many years, the luminous intensity produced by a certain size candle made from the oil of sperm whales sarved as the standard.

The unit called a candle. However, the sperm whale candle did not provide an easily used standards for the measurement of light. One candela is now defined as the amount if light given off by a source emitting at a specific frequency (540,000,000,000,000 hertz) and at a specific intensity (1/683 watt per unit of area called a steradian).

The Speed of Light Although light seems to travel across a room the instant a window shade is raised, it actually takes some time to travel any distance. The speed of light in empty space where atoms do not delay its travel-is 186,282 miles (299, 792 kilometers) per second. This speed is said to be invariant because it does not depend on the motion of the light’s source. For example, light that is emitted by a rapidly moving flashlight has the same speed as light that is emitted by a stationary flashlight. Scientists do not know why this is true, but the fact is one of the foundations of Einstein’s theory of relativity.

Interference

Diffraction

Dispersion

Polarization