Characteristics of LightNewton proposed the particle theory of light to explain the bending of light uponreflection from a mirror or upon refraction when passing from air into water. In hisview, light was a stream of particles emitted from a light source, entering the eye tostimulate sight. Newtons contemporary Christiaan Huygens showed that a wavetheory of light could explain the laws of reflection and refraction. In the late 1800s,James Clerk Maxwell predicted, and then Gustav Ludwig Hertz verified, the existence ofelectromagnetic waves traveling at the speed of light. A complete conceptualization ofthe nature of light includes light as a particle, as a wave, and as electromagneticradiation.The modern view is that light has a dual nature. To debate whether light is a particle ora wave is inappropriate because in some experiments light acts like a wave and inothers it acts like a particle. Perhaps it is most accurate to say that both waves andparticles are simplified models of reality and that light is such a complicatedphenomenon that no one model from our common experience can be devised to explainits nature.Electromagnetic spectrumMaxwells equations united the study of electromagnetism and optics. Light is therelatively narrow frequency band of electromagnetic waves to which our eyes aresensitive. Figure 1 illustrates the spectrum of visible light. Wavelengths are usuallymeasured in units of nanometers (1 nm = 10−9 m) or in units of angstroms (1Å =10−10m). The colors of the visible spectrum stretch from violet, with the shortest length,to red, with the longest wavelength. Figure 1 The spectrum of electromagnetic radiation, which includes visible light.
Speed of lightLight travels at such a high speed, 3 × 108 m/sec, that historically it was difficult tomeasure. In the late 1600s, Claus Roemer observed differences in the period of themoons of Jupiter, which varied according to the position of the earth. He correctlyassumed a finite speed of light. He deduced the annual variation was due to a changeddistance between Jupiter and the earth; so a longer period indicated that the light hadfarther to travel. His estimate, 2.1 × 108 m/s, based on his value for the radius of theearths orbit, was inaccurate, but his theories were sound. Armand Fizeau was the firstto measure the speed of light on the earths surface. In 1849, he used a rotatingtoothed wheel to find a close approximation of the speed of light, 3.15 × 108 m/s. Asshown in Figure 2 , a light beam passed through the wheel, was reflected by a mirror adistance ( d) away, and then again passed through an opening between cogs. Figure 2 Fizeaus apparatus for measuring the speed of light.Assume the speed of the wheel is adjusted so that the light passing through theopening a then passes through opening b after reflection. If the toothed wheel spins atan angular velocity ω and the angle between the two openings is θ, then the time forlight to travel 2 d isand so the velocity of light may be calculated fromwhere c denotes the speed of light. More modern methods with lasers have mademeasurements accurate to at least nine decimal places.
PolarizationLight and other elecromagnetic radiation can be polarized because the waves aretransverse. An oscillatory motion perpendicular to the direction of motion of the wave isthe distinguishing characteristic of transverse waves. Longitudinal waves, such assound, cannot be polarized. Polarized light has vibrations confined to a single planethat is perpendicular to the direction of motion. A beam of light can be represented by asystem of light vectors. In Figure3 , unpolarized light is radiating from a light bulb. Thebeam going to the top of the page is viewed along the direction of motion (as end-on).The vectors in the beam traveling to the side of the page are seen perpendicular to thedirection of motion (as a side view). Figure 3 A light bulb emits unpolarized light.Light is commonly polarized by selective absorption of a polarizing material. Tourmalineis a naturally occurring crystal that transmits light in only one plane of polarization andabsorbs the light vectors in other polarization planes. This type of material is calleda dichroic substance. A mechanical analogy illustrates this process. Imagine a ropewith transverse pulses passing through two frames of slots, as shown in Figure 4 .When the second polarizer is turned perpendicular to the first, the wave energy isabsorbed. Figure 4 A mechanical analogy of polarization, for a wave on a string.
Polaroid, another dichroic substance, is manufactured from long-chain hydrocarbonswith alignment of the chains. As you will recall, electromagnetic waves are crossedelectric and magnetic fields propagating through space. The orientation of the electricwave is taken as the direction of polarization. The polaroid molecules can conductelectric charges parallel to their chains; therefore, hydrocarbon molecules in polaroidfilters absorb light with an electric field parallel to their length and transmit light withelectric field perpendicular to their length.Figure 5 shows the direction of light vectors for a beam of light traveling through twopolaroids. The first polaroid is called the polarizer, and the second polaroid is calledan analyzer. When the transmission axes of the polarizing materials are parallel, thepolarized light passes through. Light is nearly completely absorbed when passingthrough two sets of polarizing materials with their transmission axes at right angles. Figure 5 A sequence of polaroids.Light can be polarized by reflection. For this reason, polaroid sunglasses are effectivefor reducing glare. Sunlight is primarily polarized parallel to the surface after reflection;therefore, the polaroids in sunglasses are oriented so that the reflected polarized light islargely absorbed.
Properties of Light: Reflection, Refraction,Dispersion, and Refractive IndicesReflection and Refraction of LightWhen light strikes an interface between two substances with different refractive indices, twothings occur. An incident ray of light striking the interface at an angle, i, measured between aline perpendicular to the interface and the propagation direction of the incident ray, will bereflected off the interface at the same angle, i. In other words the angle of reflection is equal tothe angle of incidence.If the second substance is transparent to light, then a ray of light will enter the substance withdifferent refractive index, and will be refracted, or bent, at an angle r, the angle of refraction.The angle of refraction is dependent on the angle of incidence and the refractive index of thematerials on either side of the interface according to Snells Law:Dispersion of LightThe fact that refractive indices differ for each wavelength of light produces an effectcalled dispersion. This can be seen by shining a beam of white light into a triangularprism made of glass. White light entering such a prism will be refracted in the prismby different angles depending on the wavelength of the light.Absorption of LightWhen light enters a transparent material some of its energy is dissipated as heatenergy, and it thus loses some of its intensity. When this absorption of energyoccurs selectively for different wavelengths of light, they light that gets transmittedthrough the material will show only those wavelengths of light that are notabsorbed. The transmitted wavelengths will then be seen as color, calledthe absorption color of the material.Polarization of LightNormal light vibrates equally in all direction perpendicular to its path ofpropagation. If the light is constrained to vibrate in only on plane, however, we saythat it is plane polarized light. The direction that the light vibrates is calledthe vibration direction, which for now will be perpendicular to the direction. Thereare two common ways that light can become polarized.
Light Sources The sun is a light source. Stars are a light source. A fire is a light source. A candle is a light source. An electric light bulb is a light source.
ELECTROMAGNETIC THEORY OF LIGHT James Clark Maxwell, a brilliant Scottish scientistOf the middle l9th century, showed, by constructing an oscillating electrical circuit,that electromagnetic waves could move through empty space. Light eventually wasproved to be electromagnetic. Current light theory says that light is made up of verysmall packets of electromagnetic energy called PHOTONS (the smallest unit ofradiant energy). These photons move at a constant speed in the medium throughwhich they travel. Photons move at a faster speed through a vacuum than they do inthe atmosphere, and at a slower speed through water than air. The electromagneticenergy of light is a form of electromagnetic radiation. Light and similar forms ofradiation are made up of moving electric and magnetic forces and move as waves.Electromagnetic waves move in a manner similar to the waves produced by the pebbledropped in the pool of water discussed earlier in this chapter. The transverse waves oflight from a light source spread out in expanding circles much like the waves in thepool. However, the waves in the pool are very slow and clumsy in comparison withlight, which travels approximately 186,000 miles per second. Light radiates from itssource in all directions until absorbed or diverted by some substance (fig.1-17). Thelines drawn from the light source (a light bulb in this instance) to any point on one of these waves indicate the direction in which the waves are moving. These lines, called radii of the spheres, are formed by the waves and are called LIGHT RAYS. Although single rays of light do not exist, light "rays" as used in illustrations are a convenient method used toshow the direction in which light is traveling at any point. A large volume of light iscalled a beam; a narrow beam is called a pencil; and the smallest portion of a pencilis called a light ray. A ray of light can be illustrated as a straight line. This straight linedrawn from a light source will represent an infinite number of rays radiating in alldirections from the source.