Characteristics of Light

Newton proposed the particle theory of light to explain the bending of light upon reflection from a mirror or upon refraction when passing from air into water. In his view, light was a stream of particles emitted from a light source, entering the eye to stimulate sight. Newton's contemporary Christiaan Huygens showed that a wave theory 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 of electromagnetic waves traveling at the speed of light. A complete conceptualization of the nature of light includes light as a particle, as a wave, and as electromagnetic radiation.

The modern view is that light has a dual nature. To debate whether light is a particle or a wave is inappropriate because in some experiments light acts like a wave and in others it acts like a particle. Perhaps it is most accurate to say that both waves and particles are simplified models of reality and that light is such a complicated phenomenon that no one model from our common experience can be devised to explain its nature.

Electromagnetic spectrum

Maxwell's equations united the study of electromagnetism and optics. Light is the relatively narrow frequency band of electromagnetic waves to which our eyes are sensitive. Figure illustrates the spectrum of visible light. Wavelengths are usually measured 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 light

Light travels at such a high speed, 3 × 10 8 m/sec, that historically it was difficult to measure. In the late 1600s, Claus Roemer observed differences in the period of the moons of Jupiter, which varied according to the position of the earth. He correctly assumed a finite speed of light. He deduced the annual variation was due to a changed distance between Jupiter and the earth; so a longer period indicated that the light had farther to travel. His estimate, 2.1 × 10 8 m/s, based on his value for the radius of the earth's orbit, was inaccurate, but his theories were sound. Armand Fizeau was the first to measure the speed of light on the earth's surface. In 1849, he used a rotating toothed wheel to find a close approximation of the speed of light, 3.15 × 10 8 m/s. As shown in Figure , a light beam passed through the wheel, was reflected by a mirror a distance ( d) away, and then again passed through an opening between cogs.


Figure 2

Fizeau's apparatus for measuring the speed of light.



Assume the speed of the wheel is adjusted so that the light passing through the opening a then passes through opening b after reflection. If the toothed wheel spins at an angular velocity ω and the angle between the two openings is θ, then the time for light to travel 2 d is

   


and so the velocity of light may be calculated from 


 


where c denotes the speed of light. More modern methods with lasers have made measurements accurate to at least nine decimal places.

Polarization

Light and other elecromagnetic radiation can be polarized because the waves are transverse. An oscillatory motion perpendicular to the direction of motion of the wave is the distinguishing characteristic of transverse waves. Longitudinal waves, such as sound, cannot be polarized. Polarized light has vibrations confined to a single plane that is perpendicular to the direction of motion. A beam of light can be represented by a system of light vectors. In Figure 3, unpolarized light is radiating from a light bulb. The beam 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 the direction 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. Tourmaline is a naturally occurring crystal that transmits light in only one plane of polarization and absorbs the light vectors in other polarization planes. This type of material is called a dichroic substance. A mechanical analogy illustrates this process. Imagine a rope with 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 is absorbed.




Figure 4

A mechanical analogy of polarization, for a wave on a string.


Polaroid, another dichroic substance, is manufactured from long‐chain hydrocarbons with alignment of the chains. As you will recall, electromagnetic waves are crossed electric and magnetic fields propagating through space. The orientation of the electric wave is taken as the direction of polarization. The polaroid molecules can conduct electric charges parallel to their chains; therefore, hydrocarbon molecules in polaroid filters absorb light with an electric field parallel to their length and transmit light with electric field perpendicular to their length.

Figure 5 shows the direction of light vectors for a beam of light traveling through two polaroids. The first polaroid is called the polarizer, and the second polaroid is called an analyzer. When the transmission axes of the polarizing materials are parallel, the polarized light passes through. Light is nearly completely absorbed when passing through 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 effective for 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 is largely absorbed.