LIGHT

WHERE LIGHT COMES FROM When a photon, or packet of light energy, is absorbed by an atom, the atom gains the energy of the photon, and one of the atom’s electrons may jump to a higher energy level. The atom is then said to be excited. When an electron of an excited atom falls to a lower energy level, the atom may emit the electron’s excess energy in the form of a photon.

LIGHT IS CALLED ELECTROMAGNETIC RADIATION Scientists have learned that all these forms of energy and many other kinds of energy, such as radio waves, microwaves, and gamma rays, have the same structure. They all consist of electrical and magnetic fields that work together in a special way to form electromagnetic radiation

ELECTROMAGNETIC SPECTRUM The electromagnetic spectrum includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma rays. Visible light, which makes up only a tiny fraction of the electromagnetic spectrum, is the only electromagnetic radiation that humans can perceive with their eyes.

POLARIZED LIGHT

Polarized light consists of individual photons whose electric field vectors are all aligned in the same direction. Ordinary light is unpolarized because the photons are emitted in a random manner, while laser light is polarized because the photons are emitted coherently. When light passes through a polarizing filter, the electric field interacts more strongly with molecules having certain orientations. This causes the incident beam to separate into two beams, whose electric vectors are perpendicular to each other. A horizontal filter, such as the one shown, absorbs photons whose electric vectors are vertical. The remaining photons are absorbed by a second filter turned 90° to the first.

LASER LIGHT

REFRACTION Light from many sources, such as the sun, appears white. When white light passes through a prism, however, it separates into a spectrum of different colors. The prism bends, or refracts, light of different colors at different angles. Red light bends the least and violet light bends the most. Light travels at different speeds in different materials. When the speed of light changes in another material, it bends.

REFRACTION Refraction is the bending of a light ray as it passes from one substance to another. The light ray bends at an angle that depends on the difference between the speed of light in one substance and the next. Sunlight reflecting off a fish in water, for instance, changes to a higher speed and bends when it enters air. The light appears to originate from a place in the water above the fish’s actual position.

DIFFRACTION When light passes through a slit with a size that is close to the light’s wavelength, the light will diffract, or spread out in waves. When light passes through two slits, the waves from one slit will interfere with the waves from the other. Constructive interference occurs when a wave front, or crest, from one wave coincides with a wave front from another, forming a wave with a larger crest. Destructive interference occurs when a wave front of one wave coincides with a trough of another, canceling each other to produce a smaller wave or no wave at all.

THE EYE

Right: The amount of light entering the eye is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. A mesh of blood vessels, the choroid, supplies the retina with oxygen and sugar. Left: Lacrimal glands secrete tears that wash foreign bodies out of the eye and keep the cornea from drying out. Blinking compresses and releases the lacrimal sac, creating a suction that pulls excess moisture from the eye’s surface.

REFRACTION: THE EYE IS A LENS THAT REFRACTS LIGHT Light rays entering the eye are refracted, or bent, when they pass through the lens. Normal vision requires that the rays focus on the retina. If the eyeball is too long, an accurately focused image falls short of the retina. This is called myopia, or near sightedness. A nearsighted person sees distant objects unclearly. Farsighted focus, or hyperopia, results when the eyeball is too short. In this case, an accurately focused image would fall behind the retina. These conditions can also occur if the muscles of the eye are unable to alter the shape of the lens to focus light rays accurately.

COLOR, ABSORBTION, and ATOMS Subtractive mixing is based on the way matter affects light. A beam of white light is changed when it meets certain kinds of matter. Some of the light stays in the matter--is absorbed. As a result, the light that emerges--is reflected or transmitted--has a different color. This happens in part because of the way matter is constructed. All matter consists of atoms. Each atom contains a dense, heavy center called a nucleus and one or more electrons that are in continuous motion around the nucleus. Distinct quantities of energy are available to each of these electrons. An electron can have the quantity of energy dictated by one or another of the atom's energy levels, but it cannot have an intermediate quantity.

WAVES

INTERFERENCE This interference pattern was formed by two rods moving rhythmically up and down in a ripple tank. A ripple tank consists of a clear tray of water, an overhead light, and devices to make wave patterns.You can observe a similar pattern by dipping two fingers up and down in a puddle of water or by watching two ducks swim near each other in a lake or pond. If two crests arrive at a point together, they superimpose to form a very high crest; if two troughs arrive together, they superimpose to form a very low trough. This is called constructive interference. The bright and dark rings are regions of constructive interference. If a crest from one source arrives at a point at the same instant as a trough from the other source, they cancel each other. This is called destructive interference. The radiating dark rays are regions of destructive interference.

INTERFERENCE When two pulses traveling on a string meet each other, the amplitudes of the pulses are added together to produce the shape of the resulting pulse. If the pulses are identical but travel on opposite sides of the string, then the sum of the amplitudes is zero and the string will appear flat for one instant (A). This is called destructive interference. When the two identical pulses travel on the same side of the string, then the sum of the amplitudes is double the amplitude of a single pulse when the pulses are together(B). This is called constructive interference.

Creating the common hologram involves the use of two beams of light originating from a single laser. This laser light is of a single frequency (monochromatic) and forms waves that are in phase with one another (coherent). The light beam originating from the laser is split by a partially reflective mirror. The narrow beams are redirected by mirrors and expanded by lens and pinhole arrangements. One beam, called the object beam, illuminates the surface of the object. The light then scatters and reflects onto the film. The other beam, called the reference beam, strikes the film directly. These two beams interact on the film, producing an interference pattern of closely spaced lines, called a complex diffraction grating. This film of the pattern is the hologram.