Alayssa Pearl M. Fazon III - Galileo. When two light waves cross through the same spot, they interfere with each other – that is, they add to or subtract.

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Alayssa Pearl M. Fazon III - Galileo

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 process called constructive interference gives brighter light than either wave would have separately. The trough reduces the height of the crest, leaving the spot dim or even dark. This process is called destructive interference.

The fact that the light can interfere to produce brightness or darkness provides the strongest argument for the wave model of light. In the early 1800’s the English scientist Thomas Young showed the wave nature of light by sending a light beam through two narrow slits. The light that emerged from the slits then reached a screen. If light were not a wave, only two narrow, bright strips of light – one from each slit – would have appeared on the screen. But, in fact, the light emerging from each slit spread and overlapped the light emerging from the other slit.

The light filled the screen with dark and bright lights are called fringes. Bright fringes occurred where the two waves arrived crest-on-crest to give constructive interference. Dark fringes occurred where the two waves arrived crest-on-trough to give destructive interference.

You can see this effect pretty easily if you hold your firdts and second finger close to the eye (close the other eye) and parallel to each other. Then while looking at a bright area (preferably not a very small bright light), squeeze your two fingers closer and closer together unbtil yo see stripes parallel to your fingers as the slit becomes very thin.

Because the amount of bending or spreading changes with the wavelength (more bending of blue waelengths and less bending of red wavelengths) of the light, if the slit is thin enough and repeated many times to make a ery fine grating, this effect can make rainbows out of white light by spreading out the many colors which make up white light. The funky glasses that make you see rainbows everywhere are made of transparent, very fine diffraction gratings. The “holographic” and “prismatic” stickers use diffraction gratings to create rainbows as well.

Where Ѳ is he angle between the central and first bright band, is the wavelength of the light, x is the distance along the screen to the first bright band, d is the distance between the two slits, and L is the distance from the slits to the screen. More patterns, which are the changing patterns you see when you overlap two transparencies with thin stripes, are a consequence of interference. You can see them when you look through two chain link fences as you pass them in a car.

In the thickness of variuos films, differet colors og light are able to constructively interfere hen the path difference between light reflected from the top surface and from the bottom surface matches the light’s wavelength, causing the changing stripes of color we see in soap bubbles. The thin films of oil on puddles, and the colored bands on a CD. A layer of thickness, t, of ink is placed o the CD surface and then ruled with concentric circles of information. As you view the reflection at larger angles, the path differece becomes larger and a longer wavelegth og light will be able to constuctively interfere as well, and more bands of color will appear.

Newton’s rings are a very precise way to measure the curvature of flatness of a piece of glass. The light reflecting from the bottom of the lens interferes with the light from the top of the glass plate, creating concentric rings. Holograms are specialized interference patterns recorded on a thin film emulsion on glass or plastic, enabling your eyes to see exactly the patter of ight waves that was reflected by a three-dimensional object.

By using ery coherent light – that is, single frequency and no variation in phase across the beam so the beam will not interfere with itself – and keeping the optical elemets very still with respect to each other, a very precise interference pattern is recorded on film. When this film is developed and viewed at the proper angle, the intereference pattern on the film recreates the light waves which were reflected by the object, giving you a sense of seeing the three-dimensional object itself, even though the object is no longer there.

In Young’s experiment, the light passing through each slit spread. This type of spreading is called diffaction. Like interference, it results from the fact that light behaves as a wave. A light wave spreads lightly when it travels through a small opening, around a small object, or past an ede. Water waves also spread, but the openings and objects that cause them to spread must be much larger that those for light.

Diffraction of light can be 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 more and more closely, the object’s edges begin to blur. Each edge blurs beacause the light passing by the edge on its way to the eye diffracts. However, diffraction serves a purpose when a device called a diffraction grating is used to study the colors in a light beam. The grating consists of thousands of thin slits that diffract light.

Each color in the light diffracts by a slightly different amount. The spread of colors can be large enough to make each color visible. A grating used with a telescope can separate the colors in the light from a star, enabling scientists to learn what materials make up the star. The spreading of a wave around the corner or through a slit width b, which is about a order of magnitude of the size of the wavelength, is called diffraction. Light that is passed through a narro slit produces a central bright band ith parallel bands to both sides which are about 1/20, the brightness of the central.

Dispersion is the spreading of light into its colors. The dispersion of white light separates the colors of the full visible spectrum. One way to disperse a light beam is to sed it through a prism. The different colors refract to different extents. Thus, the colors spread. Diffraction and scattering can also disperse light. A ray of white light that passes through a prism is disperse into the visible spectrum. Red light is refracted the least, and purple light is refracted the most. This is bacause the speed of the various wavelength in glass is different, slowest for violet light and fastest for red light.

Originally, prisms were used in machines called monochromators to spread the spectrum of light coming from the stars. Now we can lose less light by using high resolution diffraction gratings to disperse the light into a rainbow. The first indication of which elements were present in stars was deduced from the spectrums obtained this way: these spectrums were observed to have dark bands in them. The dark bands were matched to absorption at the same wavelengths by elements on earth, and we were able to tell what elements were present in a given star. As the wavelength of these absorption bands shifted, we were able to measure Doppler shifts and the speed of the stars towards or away from Earth.

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 represented by arrows. In most of the light we see, the arrows point in many directions perpindecular to the ray’s path. Such light is unpolarized. But few of the arrows remain when light passes through certain types of sunglasses, reflects from surfaces at certain angles, or scatters from air molecules.

If these arrows all point in one direction or just opposite it, the light is polarized. Suppose that when sunlight reflects from a road to you, its arrows point only to your left or right. You can block it by wearing sunglasses with polarizing filters. They block light oscillating left or right.

The energy of light can chemically change the surfaces of materials absorbing it. For example, light chemicaly changes the molecules of silver grains on photographic film so hat an image can be recorded on it. Strong light can fade colored fabrics by chemically changing their dyes. Light hanges the chemistry of the eye’s retina, so that the retina prouces signals about sight. Green plants need light for photosynthesis, the chemical process by which they make food.

When certain materials absorb light, the light’s energy frees electrons from atoms on the materials’ surface. In some devices, these freed electrons can flow through a circuit as electric current. Solar cells and other electric eyes operate by means of such photoelectric effects. Some materials called photoconductors become better conductors of electricity when light shines on them.

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 of light in the visible spectrum range from about 400 nanometers for deep violet to about 700 nanometers for deep red. The frequency of any wave equals the ratio of the wave’s speed to its wavelength. Frequencies are measured in units called hertz.

A wave has a frequency of one hertz if one crest passes a checkpoint each second, and the ave has a frequency of 100 hertz if 100 crests pass a checkpoint each second. Light travels in a vacuum at nearly 300 million meters per second. Because visible light has ashort wavelength and high speed, it has a high frequency. For example, violet light has a frequency of 750 trillion hertz.

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 served as the standard.

The unit was called a candle. However, the sperm whale candle did not proide an easily used standard for the measurement of light. One candela is now defined as the amount of light gien 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 intensity of light source in candelas does not indicate how bright the light will be when it reaches the surface of an object, such as a book or a desk.

Before we can measure illumination (the light falling on a surface), we must measure the light traveling through the space between the source and the object. We can measure a beam of light with a unit called the lumen. To see how the lumen is measured, imagine a light source placed at the center of ahollo sphere. On the inside surface of the sphere, an area is marked off equal to the square of the readius of the sphere. For example, if the radius is 1 foot, the area marked off is 1 square foot. If the light source has a luminous intensity of 1 candela, the marked area will receive a luminous flux (rate of light falling on it) of 1 lumen.

The intensity of light faling on a surface varies inversely (oppositely) with the square of the distance between the source and the surface. That is, if the distance increases, the illumination decreases by the square of the distance. This relationship is called the inverse square law. If the surface that receives 1 lux of light at a distance of 1 meter from a source is moved 2 meters from the source, that surface will then receive 1/2squared or 1/4, lux of light. This happens because light spreads out from its source.

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 fundations of Einstein’s theory of relativity.

From ancient times, people argued about whether the speed of light is limited or infinite. During the early 1600’s the Italian physicist Galileo devised an experiment to measure the speed of light, and to settle the arguement. Galileo sent an assistant to a distant hill ith instructions that the assistant should open the shutter of a lantern when he saw Galileo on another hill open the shutter of his lantern. Galileo reasoned that because he knew the distance between the hills, he could find the elocity of light by measuring the time between opening his shutter and seeing the light of the second lantern. Galileo’s thinking was sound, but the experiment failed. The velocity of light is so great that he could not measure the short time involved.

About 1675, the danish astronomer Olaus Roemer came upon evidence which proved that lighty travels at a finite (limited) speed. While working in paris, Roemer obeserved that the intervals between the disappearances of some of Jupiter’s moons behind the planet varied with the changing distance between Jupiter and Earth. Roemer realized that the finite velocity of light caused these differing intervals. Roemer’s obserations indicated that light traveled at a speed of 226,000 kilometers per second. This figure was within 25 percent of the actual velocity.

In 1926, the American physicist Albert A. Michelson made one of the first precise measurements of the velocity of light. He used a rapidly rotating mirror that reflected a beam of light to a distant reflector. The returning beam was then reflected back to the observer by the rotating mirror. Michelson adjusted the speed of the mirror until the mirror turned to the correct angle during the time the light traveled to the reflector and back. The speed of the mirror indicated the velocity of the light. Michelson actually used several mirrors on a drum so that the angle of the drum have to turn while the light traveled out and back was small. He measured the speed of light at 299,796 kilometers per second. This measurement ha a probable error of less than 4 kilometers per second.

A Dutch mathematician, born in Leiden, and taught at the University of Leiden. Snell became famous for formulating the law of refraction. The equation of refraction is also called Snell’s Law. He also tried to measure the circumference of the earth by triangulation. His measurements were inaccurate.

Born in Germany but later became an American citizen, he and physicist Edward Morley became famous for the experiment that calculated the speed of light. Through this he won the Nobel Prize in 1907, the first American bestowed with this award. With Morley, they also gave the breakthrough that disproved the existence of ether, which was believed to the medium occupying space.

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