Light – introduction
Until the beginning of the 19th century, light was modeled as a stream of articles emitted by a source that stimulated the sense of sight on entering the eye. The chief architect of the particle theory of light was Newton. With this theory, he provided simple explanations of some known experimental facts concerning the nature of light—namely, the laws of reflection and refraction.
Most scientists accepted Newton’s particle theory of light. During Newton’s lifetime,
however, another theory was proposed. In 1678, the Dutch physicist and astronomer
Christian Huygens (1629–1695) showed that a wave theory of light could also
explain the laws of reflection and refraction.
The wave theory didn’t receive immediate acceptance, for several reasons. First, all the waves known at the time (sound, water, and so on) traveled through some sort of medium, but light from the Sun could travel to Earth through empty space. Further, it was argued that if light were some form of wave, it would bend around obstacles; hence, we should be able to see around corners. It is now known that light does indeed bend around the edges of objects. This phenomenon, known as diffraction, is difficult to observe because light waves have such short wavelengths. Even though experimental evidence for the diffraction of light was discovered by Francesco Grimaldi (1618–1663) around 1660, for more than a century most scientists rejected the wave theory and adhered to Newton’s particle theory, probably due to Newton’s great reputation as a scientist.
The first clear demonstration of the wave nature of light was provided in 1801 by Thomas Young (1773–1829), who showed that under appropriate conditions, light exhibits interference behavior. Light waves emitted by a single source and traveling
along two different paths can arrive at some point and combine and cancel each other by destructive interference. Such behavior couldn’t be explained at that time by a particle model, because scientists couldn’t imagine how two or more particles could come together and cancel each other.
The most important development in the theory of light was the work of Maxwell, who predicted in 1865 that light was a form of high-frequency electromagnetic wave. His theory also predicted that these waves should have a speed of 300 000 km/s, in agreement with the measured value. Although the classical theory of electricity and magnetism explained most known properties of light, some subsequent experiments couldn’t be explained by the assumption that light was a wave. The most striking of these was the photoelectric effect discovered by Hertz. Hertz found that clean metal surfaces emit charges when exposed to ultraviolet light.
In 1905, Einstein published a paper that formulated the theory of light quanta (“particles”) and explained the photoelectric effect. He reached the conclusion
that light was composed of corpuscles, or discontinuous quanta of energy. These corpuscles or quanta are now called photons to emphasize their particlelike nature. According to Einstein’s theory, the energy of a photon is proportional to the frequency of the electromagnetic wave associated with it, or:
E = h f
where h=6.63 x 10ˆ(-34) J.s is Planck’s constant. This theory retains some features of both the wave and particle theories of light. As we will discuss later, the photoelectric effect is the result of energy transfer from a single photon to an electron in the metal. This means the electron interacts with one photon of light as if the electron had been struck by a particle. Yet the photon has wavelike characteristics, as implied by the fact that a frequency is used in its definition.
In view of these developments, light must be regarded as having a dual nature : In some experiments light acts as a wave and in others it acts as a particle. Classical electromagnetic wave theory provides adequate explanations of light propagation and of the effects of interference, whereas the photoelectric effect and other
experiments involving the interaction of light with matter are best explainedby assuming that light is a particle.
So in the final analysis, is light a wave or a particle? The answer is neither and both: light has a number of physical properties, some associated with waves and others with particles.
Light originates from the accelerated motion of electrons. It is an electromagnetic phenomenon and only a tiny part of a larger whole – a wide range of electromagnetic waves called the electromagnetic spectrum.
If you shake the end of a stick back and forth in still water, you will produce waves on the surface of the water, Similarly, if you shake an electrically charged rod to and fro in empty space, you will produce electromagnetic waves in space. This is because the moving charge is actually an electric current. What surrounds an electric current? The answer is, a magnetic field. What surrounds a changing electric current? The answer is, a changing magnetic field. Recall that a changing magnetic field generates an electric field- electromagnetic induction. If the magnetic field is oscillating, the electric field that it generates will be oscillating, too. And what does an oscillating electric field do? It induces an oscillating magnetic field. The vibrating electric and magnetic fields regenerate each other to make up an electromagnetic wave, which emanates (moves outward) from the vibrating charge. There is only one speed, it turns out, for which the electric and magnetic fields remain in perfect balance, reinforcing each other as they carry energy through space. But why is this so?
Electromagnetic Wave Velocity
A spacecraft cruising through space may gain or lose speed, even if its engines are shut off, because gravity can accelerate it forward or backward. But an electromagnetic wave traveling through space never changes its speed. Not because gravity doesn’t act on light, for it does. Gravity can change the frequency of light or deflect light, but it can’t change the speed of light. What keeps light moving always at the same, unvarying speed in empty space? The answer has
to do with electromagnetic induction and energy conservation. If light were to slow down, its changing electric field would generate a weaker magnetic field, which, in turn, would generate a weaker electric field, and so on, until the wave dies out. No energy would be transported from one place to another, So light cannot travel slower than it does. If light were to speed up, the changing electric field would generate a stronger magnetic field, which, in turn, would generate a stronger electric field, and so on, a crescendo of ever-increasing field strength and ever- increasing energy-clearly a no-no with respect to energy conservation. At only one speed does mutual induction continue indefinitely, carrying energy forward without loss or gain. From his equations of electromagnetic induction, James Clerk Maxwell calculated the value of this critical speed and found it to be 300,000 kilometers per second, In his calculation, he used
only the constants in his equations determined by simple laboratory experiments with electric and magnetic fields. He didn’t use the speed of light. He found the speed of light! Maxwell quickly realized that he had discovered the solution to one of the greatest mysteries of the universe-the nature of light. He discovered that light is simply electromagnetic radiation within a particular frequency range, 4.3 x 10^ 14 to 7 x 10^ 14 vibrations per second. Such waves activate the “electrical antennae” in the retina of the eye. The lower-frequency waves appear red, and the higher-frequency waves appear violet. Maxwell realized, at the same time, that electromagnetic radiation of any frequency propagates at the same speed as light.
The Electromagnetic Spectrum
In a vacuum, all electromagnetic waves move at the same speed and differ from one another in their frequency. The classification of electromagnetic waves according to frequency is the electromagnetic spectrum. Electromagnetic waves have been detected with a frequency as low as 0.01 hertz (Hz). Electromagnetic waves with frequencies of several thousand hertz (kHz) are classified as very low frequency radio waves. One million hertz (MHz) lies in the middle of the AM radio band. The very high frequency (VHF) television band of waves starts at about 50 MHz, and FM radio waves are between 88 and 108 MHz. Then come ultrahigh frequencies (UHF), followed by microwaves, beyond which are infrared waves, often called “heat waves.” Further still is visible light, which makes up less than 1 millionth of 1 % of the measured electromagnetic spectrum. The lowest frequency of light visible to our eyes appears red. The highest frequencies of visible light, which are nearly twice the frequency of red light, appear violet. Still higher frequencies are ultraviolet. These higher-frequency waves cause sunburns. Higher frequencies beyond ultraviolet extend into the X-ray and gamma-ray regions. There are no sharp boundaries between the regions, which actually overlap each other. The spectrum is separated into these arbitrary regions for classification.
Recall that the frequency of a wave is the same as the frequency of the vibrating source. The same is also true for EM waves: the frequency of an electromagnetic wave as it vibrates through space is identical to the frequency of the oscillating electric charge generating it. Different frequencies correspond to different wavelengths-waves of low frequency have long wavelengths and waves of high frequencies have short wavelengths. For example, since the speed of the wave is 300,000 kilometers per second, an electric charge oscillating once per second (1 hertz) will produce a wave with a wavelength of 300,000 kilometers. This is because only one wavelength is generated in 1 second. If the frequency of oscillation were 10 hertz, then 10 wavelengths would be formed in 1 second, and the corresponding wavelength would be 30,000 kilometers. A frequency of 10,000 hertz would produce a wavelength of 30 kilometers. So the higher the frequency of the vibrating charge, the shorter the wavelength of radiant energy.
We tend to think of space as empty, but only because we cannot see the montages of electromagnetic waves that permeate every part of our surroundings. We see some of these waves, of course, as light. These waves constitute only a microportion of the electromagnetic spectrum. We are unconscious of radio waves, which engulf us every moment, Free electrons in every piece of metal on the Earth’s surface continually dance to the rhythms of these waves. They jiggle in unison with the electrons being driven up and down along radio- and television- transmitting antennae. A radio or television receiver is simply a device that sorts and amplifies these tiny currents. There is radiation everywhere. Our first impression of the universe is one of matter and void, but actually the universe is a dense sea of radiation in which occasional concentrates are suspended.
Light is an energy-carrying electromagnetic wave that emanates from vibrating electrons in atoms. When light is transmitted through matter, some of the electrons in the matter are forced into vibration, In this way, vibrations in the emitter are transmitted to vibrations in the receiver. This is similar to the way sound is transmitted.
Thus the way a receiving material responds when light is incident upon it depends on the frequency of the light and on the natural frequency of the electrons in the material. Visible light vibrates at a very high frequency, some 100 trillion times per second (10^14 hertz). If a charged object is to respond to these ultrafast vibrations, it must have very, very little inertia. Because the mass of electrons is so tiny, they can vibrate at this rate.
Such materials as glass and water allow light to pass through in straight lines. We say they are transparent to light. To understand how light travels through a transparent material, visualize the electrons in the atoms of transparent materials as if they were connected to the nucleus by springs. When a light wave is incident upon them, the electrons are set into vibration.
Materials that are springy (elastic) respond more to vibrations at some frequencies than at others. Bells ring at a particular frequency, tuning forks vibrate at a particular frequency, and so do the electrons of atoms and molecules. The natural vibration frequencies of an electron depend on how strongly it is attached to its atom or molecule, Different atoms and molecules have different “spring strengths.” Electrons in the atoms of glass have a natural vibration frequency in the ultraviolet range. Therefore, when ultraviolet waves shine on glass, resonance occurs and the vibration of electrons builds up to large amplitudes, just as pushing someone at the resonant frequency on a swing builds to a large amplitude. The energy any glass atom receives is either reemitted or passed on to neighboring atoms by collisions, Resonating atoms in the glass can hold onto the energy of the ultraviolet light for quite a long time (about 100 millionths of a second). During this time, the atom makes about 1 million vibrations, and it collides with neighboring atoms and gives up its energy as heat. Thus, glass is not transparent to ultraviolet light.
At lower wave frequencies, such as those of visible light, electrons in the glass atoms are forced into vibration, but at lower amplitudes. The atoms hold the energy for a shorter time, with less chance of collision with neighboring atoms, and with less energy transformed to heat. The energy of vibrating electrons is reemitted as light. Glass is transparent to all the frequencies of visible light. The frequency of the reemitted light that is passed from atom to atom is identical to the frequency of the light that produced the vibration in the first place. However, there is a slight time delay between absorption and reemission.
It is this time delay that results in a lower average speed of light through a transparent material. Light travels at different average speeds through different materials, We say average speeds because the speed of light in a vacuum, whether in interstellar space or in the space between molecules in a piece of glass, is a constant 300,000 kilometers per second. We call this speed of light c. The speed of light in the atmosphere is slightly less than in a vacuum, but it is usually rounded off as c. In water, light travels at 75% of its speed in a vacuum, or 0.75 c. In glass, light travels at about 0.67 c, depending on the type of glass. In a diamond, light travels at less than half its speed in a vacuum, only 0.41 c. When light emerges from these materials into the air, it travels at its original speed, c.
Infrared waves, with frequencies lower than those of visible light, vibrate not only the electrons, but entire atoms or molecules in the structure of the glass, This vibration increases the internal energy and temperature of the structure, which is why infrared waves are often called heat waves. So we see that glass is transparent to visible light, but not to ultraviolet and infrared light.
Different materials have different molecular structures and therefore absorb or reflect light from various spectral ranges differently.
Most things around us are opaque-they absorb light without reemIttmg it. Books, desks, chairs, and people are opaque. Vibrations given by light to their atoms and molecules are turned into random kinetic energy-into internal energy. They become slightly warmer.
Metals are opaque, Because the outer electrons of atoms in metals are not bound to any particular atom, they are free to wander with very little restraint throughout the material (which is why metal conducts electricity and heat so well). When light shines on metal and sets these free electrons into vibration, their energy does not “spring” from atom to atom in the material but, instead, is reflected. That’s why metals are shiny.
Earth’s atmosphere is transparent to some ultraviolet light, to all visible light, and to some infrared light, but it is opaque to high-frequency ultraviolet light. The small amount of ultraviolet that does get through is responsible for sunburns. If it all got through, we would be fried to a crisp. Clouds are semitransparent to ultraviolet, which is why you can get a sunburn on a cloudy day. Dark skin absorbs ultraviolet before it can penetrate too far, whereas it travels deeper in fair skin. With mild and gradual exposure, fair skin develops a tan and increases protection against ultraviolet. Ultraviolet light is also damaging to the eyes-and to tarred roofs. Now you know why tarred roofs are covered with gravel.
Have you noticed that things look darker when they are wet than they do when they are dry? Light incident on a dry surface bounces directly to your eye, while light incident on a wet surface bounces around inside the transparent wet region before it reaches your eye. What happens with each bounce? Absorption! So more absorption of light occurs in a wet surface, and the surface looks darker.
Insights: longer-wavelength ultraviolet, called UV-A, is close to visible light and isn’t harmful. Short-wavelength ultraviolet, called UV-C, would be harmful if it reached us, but is almost completely stopped by the atmosphere’s ozone layer. It is the intermediate ultraviolet, UV-B, that can cause eye damage, sunburn, and skin cancer.
Insights: Metals are shiny because light that shines on them forces free electrons into
vibration, and these vibrating electrons then emit their “own” light waves as reflection.