Biographies & Memoirs

Chapter 7

And There Was Light

In This Chapter

bullet Looking at early attempts to measure the speed of light

bullet Measuring the speed of light accurately

bullet Realizing that light is an electromagnetic wave

bullet Considering Newton’s theory of colors

bullet Appreciating Young’s experiment

W hat is light? Scientists have been trying to understand the nature of light for centuries. The Greeks thought that light was made up of small particles traveling in straight lines that entered our eyes and stimulated our sense of vision. Isaac Newton also thought that light was made up of little particles. But the English scientist Thomas Young thought that light was a wave, and he came up with a clever experiment to prove it.

In Chapter 16, I discuss Einstein’s view on the nature of light. But before I can delve into how Einstein revolutionized our understanding of light, I need to explain what Einstein learned about light from his teachers and his own studies. In this chapter, I present a quick overview of the study of light from the time of Galileo, and I discuss Newton’s and Young’s contributions to the field.

Trying to Measure the Speed of Light

As I explain in Chapter 6, James Clerk Maxwell’s theory of electromagnetism tells us that light is an electromagnetic wave traveling at 300,000 kilometers per second (kps). Maxwell’s equations tell us that changing electric and magnetic fields create and sustain each other even in regions where there are no electric charges to accelerate or magnets to move. Maxwell showed how these two fields, interlocked in a dance, create their own light show. The fields spread out through space as light or as any other electromagnetic wave.

But before Maxwell, other scientists made their own attempts to identify the nature of light and to calculate its speed. In this section, I discuss two such attempts that Einstein learned about in school.

Galileo: Hanging lanterns

How fast does light travel through space? The modern value for the speed of light is 300,000 kps (186,000 mps). Actually, it’s 299,792.458 kps, but that’s a tough number to remember. The circumference of the earth is about 40,000 km (25,000 mi), so it would take light slightly longer than a tenth of a second to travel around the world.

With modern instruments, the extremely large value of the speed of light can be measured. But how did anyone measure it before those instruments were created?

Galileo was the first person who tried. He had two people stand on distant hills flashing lanterns. Clearly, Galileo’s experiment didn’t work — he couldn’t even measure seconds accurately, much less the tiny fraction of a second that it took for light to travel between the two hills.

But Galileo was Galileo, and with his crude approach to this very difficult experiment, he was still able to show that the speed of light is finite. His contemporary, French philosopher René Descartes, had been saying that it was infinite.

Roemer: Timing a satellite

Some 70 years after Galileo’s experiment, the young Danish astronomer Olaus Roemer was able to get the first value of the speed of light. But he had to go farther than a distant hill to get it. He used the satellites of Jupiter instead. And he also had to fight with his boss — the famous astronomer Jean-Dominique Cassini, for whom the Saturn rings are now named.

Tackling an inconsistency

Roemer was a bright 21 year old who was hired by one of Cassini’s assistants to help at the Paris Observatory, which was headed by Cassini. But Roemer didn’t just help; he tackled one of the observatory’s major problems.

Cassini’s observations were showing a problem with the motion of one of Jupiter’s satellites, the one named Io (after one of the many lovers of Zeus, who is called Jupiter in Roman mythology). It seemed as if Io’s orbit was a bit unpredictable. The times when the satellite came out from behind the planet changed inexplicably. Cassini ordered his assistants to make better observations and to do more calculations.


Roemer doubted that the observations or calculations were the problem. The problem was that no one had taken into account the relative distance of the Earth and Jupiter as the two planets went around the sun. At different places in their orbits, the planets are sometimes closer and sometimes farther apart. When Io comes out from behind Jupiter, the distance that light travels from the satellite to the Earth depends on the separation of the planets at that time.

Cassini didn’t agree with his assistant. He believed that light traveled from place to place instantaneously, without delays. It didn’t matter how far Jupiter was.

Roemer stuck with his idea. He went back and reviewed many years’ worth of data taken in Cassini’s observatory. With this data, he was able to calculate the changes in the eclipsing times for Io as it went around in its orbit. He was sure that he was right and wanted to go public.

Going around the boss

What to do? Normally, the lab director would make the public presentation of new findings, along with the researcher who made the discovery. But Cassini didn’t agree with Roemer’s work, so Roemer decided to go alone. He’d been in Cassini’s observatory for five years and felt cocky. He appeared before the Academy of Sciences in Paris and announced that Io was going to come out from behind Jupiter exactly ten minutes after Cassini said it would.

Cassini had calculated that Io was going to come out of the eclipse on November 9, 1676, at 5:25:45. The astronomers went out to look that night. 5:25:45 came and went, and Io wasn’t there. At 5:30, there were still no signs of it. But at 5:35:45, Io reappeared. Roemer had been right.

Roemer’s friend Christian Huygens used this data to come up with the first measured value for the speed of light. His number was 227,000 km (140,000 mi) per second, which is about 24 percent lower than the modern value.

Cassini never admitted his error. Most European astronomers followed Cassini and didn’t believe that the speed of light was finite. Some 50 years later, other methods to measure the speed of light showed that Roemer had been correct.

By the time Einstein was in school, the speed of light had been measured with fairly good accuracy. This speed, represented in Einstein’s work by the letter c, ended up at the very foundation of his special theory of relativity.


How long does it take?

You can get a feel for the speed of light and for the distances in the universe with the examples shown in the following table. Einstein said that nothing can travel at the speed of light except for light itself, so you would take much, much longer to get to these destinations. But maybe not as long as you’d think. In Chapter 10, I discuss how Einstein’s special theory of relativity helps us get around the slow pace of travel we’re used to.

Traveling at the Speed of Light


Time for Light to Get



Across the room

0.02 millionths of a




A bit over 1 second


8 minutes

Edge of the solar system

51/2 hours

Nearest star (Alpha Centauri)

4 years

Center of the galaxy

30,000 years

Across the Milky Way galaxy

100,000 years

Nearest galaxy (Andromeda)

1 million years


Proving Maxwell Right

James Clerk Maxwell died of cancer at the age of 48. Because he died so young, he never saw the confirmation of his discovery that light was an electromagnetic wave that could be generated by moving a charge back and forth. If he’d lived just eight more years, he would’ve seen the proof.

It fell to Heinrich Hertz to produce Maxwell’s electromagnetic waves and measure their speed. Hertz was a bright German physicist who had obtained his PhD at age 23 from the University of Berlin. Three years later, in 1883, he decided to study Maxwell’s papers so he could understand the theory of electromagnetism, which wasn’t yet taught in college or graduate school. (It was too new, and few professors knew enough about it to teach it.)

Jumping sparks

Hertz learned electromagnetism on his own, and in two years, he was an expert. He decided to generate the electromagnetic waves Maxwell had described and to measure their speed.

He built a replica of the setup that Michael Faraday had used to discover his famous induction law (see Chapter 6), which became one of Maxwell’s equations. But Hertz modified it a bit. Faraday had wrapped two separate insulated wires around a metal ring: One wire was connected to a battery, and the other was a closed loop. Hertz decided to open the closed loop and place two small metal balls at the ends of the wire (see Figure 7-1). The two balls were separated by a small gap.

Figure 7-1: Hertz’s experimen- tal setup with two small metal balls separated by a small gap.

Figure 7-1: Hertz’s experimen- tal setup with two small metal balls separated by a small gap.

From Maxwell’s theory of electromagnetism, Hertz knew that when he connected or disconnected the battery in the first coil, the fast change in current would produce a changing magnetic field that in turn would generate a voltage in the second separate coil. In other words,

Changing electric current changing magnetic field voltage in second coil

When the voltage generated in the open coil was large enough, a spark jumped across the two balls. According to Maxwell’s theory, these sparks sent changing electric and magnetic fields across the gap, which should take off and move across the surrounding space as an electromagnetic wave.

Inventing the first radio


Hertz realized that if he had a second open loop with balls at the ends forming a small gap, the electromagnetic field would reach it and produce a voltage in that new loop (see Figure 7-2).

Figure 7-2: Hertz’s invention of the radio transmitter and receiver.

Figure 7-2: Hertz’s invention of the radio transmitter and receiver.

Hertz had just invented a radio transmitter (the first set of loops) and a radio receiver (the second loop). He was then able to measure the speed of electromagnetic waves generated with this spark setup. He got the same value as the speed of light, exactly as Maxwell had said.

Making light

Hertz’s experiment showed that light was an electromagnetic wave. Hertz had essentially made light by running an electric current through a wire. This light, which was invisible, was actually a radio wave. What do radio waves and light have to do with each other? Radio waves, light, and other waves that were discovered later are all electromagnetic waves generated in a similar way — by accelerating electric charges. The only difference among them is how fast the waves wave, or oscillate.


Once generated, all electromagnetic waves spread out through space in all directions at the speed of light, 300,000 kps. You can imagine them as a pulsating bubble rapidly expanding, as shown in Figure 7-3.

Figure 7-3: Electromag-netic waves spread out in all directions, like an expanding bubble.

Figure 7-3: Electromag-netic waves spread out in all directions, like an expanding bubble.

If you examine an electromagnetic wave with some instruments, you detect the electric and magnetic fields pulsating or oscillating in step (see Figure 7-4). This oscillation is what travels through space. The wave doesn’t deform as it travels; the length of each oscillation, or wavelength, stays the same for a particular electromagnetic wave. But the wavelength is different for different waves. Radio waves, like the ones generated by Hertz, have wavelengths that range from about 1 meter to thousands of kilometers. Hertz’s was a short one, about 1 meter in length.

Figure 7-4: The pulsating electric and magnetic fields that make an electromag- netic wave oscillate in step.

Figure 7-4: The pulsating electric and magnetic fields that make an electromag- netic wave oscillate in step.


The electromagnetic waves that heat up your food in your microwave are measured in centimeters, and x-rays are much, much smaller. That’s why they can penetrate through your skin and muscles and give physicians a picture of your bones.

Identifying the electromagnetic spectrum

Visible light has wavelengths larger than x-rays but smaller than those for radio and TV. Because of the small size of the wavelengths, scientists use nanometers to designate their length — a nanometer (nm) is a millionth of a millimeter. Visible light ranges from 400 nm for the color red to 700 nm for violet.

In Figure 7-5, which shows the electromagnetic spectrum (as Newton called it), I list the names we’ve given to the different ranges of wavelengths discovered. Each wavelength has a different energy; the shorter wavelengths are more energetic. Scientists have invented instruments to detect different ranges, like x-rays, gamma rays, or radio.

Figure 7-5: The electro- magnetic spectrum.

Figure 7-5: The electro- magnetic spectrum.

Creating a Theory of Colors

During his miracle year of 1666 (see Chapter 4), Newton started experiments on what later became his theory of colors. This work was of great importance; what we know today about color started with the experiments he did in that marvelous year.

Newton knew that a beam of light passing through a prism broke up into a splash of the colors of the rainbow: violet, blue, green, yellow, orange, and red. This knowledge was commonplace even at the time of Aristotle. But no one knew why light could be broken up into colors until Newton came along.

Drilling a hole in the shutters

Early in 1666, Newton wanted to try the “celebrated phenomena of colors.” He bought a glass prism, brought it home to his mother’s farm, went into his room, closed the doors and windows, and made a small hole in the shutters to let a narrow beam of light come into the room.


Red curtains

Newton’s favorite color was red. When he died, people were surprised to find that this serious, conservative man was surrounded in his home by a sea of red. His bed was covered with a red bedspread, and red drapes with matching red valances covered the windows. In his dining room, he had a special red chair for the occasional guest. In his parlor, he had a red easy chair and six red cushions where he sat to read or rest after he returned home from work.

Newton didn’t grow up in a very colorful environment. The clothes that he wore growing up were gray, brown, or tan. But every once in a while, someone would gather some bright berries and roots to dye clothes for special occasions. Newton was very interested in these dyes, especially the red ones. In one of the notebooks that he kept about things that interested him, Newton had different recipes for preparing painters’ colors, with many more formulas for making red than for any other color.

This early interest in color appears in contrast with Newton’s lack of interest in the beauty of nature. Unlike many scientists, Newton rarely showed any interest in anything other than the abstract concepts of physics and mathematics.

Did Newton’s interest in mixing colors and pigments as a young boy influence his scientific interest in them later? There is no evidence to support this idea. But there is no question that he had an intense scientific interest in color.


He placed his prism in the path of the light beam. The beam broke out into colors on the opposite wall. People had seen this happen many times — it was nothing new, but it was beautiful.

Newton noticed something peculiar. He had carefully made a circular hole, but the shape of the spot on the wall was oval, not circular.

Before Newton, people thought that a prism changed the color of light. The theory was that sunlight passing through the thick end of a prism was darkened more, so it became blue, while light passing through the thin end was darkened less and became red. But the prevailing theory didn’t explain why the round hole made an oval shape on the wall.

Newton wanted to known why. He made the hole bigger, then smaller. He changed the location of the prism and the place where the beam entered the prism. The spectrum never changed.

Newton placed a second prism a few yards away so that the light beam would pass through both prisms. Then, he noticed something remarkable. The blue end of the rainbow was bent even farther than the red, but no additional colors appeared. The second prism didn’t change the color of light at all: Red was still red, and blue was still blue.


This experiment was key because Newton discovered that light isn’t changed by the prisms. It is, instead, separated into different colors. After these colors are separated by one prism, they can’t be separated any further.


Millions of colors

White light is made up of the colors of the rainbow. How many colors are there? You may hear people say that there are only seven colors in the rainbow, but you know that there are many other colors. You can set up your computer monitor to display “millions of colors,” for example. Where do these colors come from?

A prism actually breaks light up into an infinite number of colors. The problem is that our eyes aren’t sensitive enough to see them all. The eye has only three kinds of cone cells that distinguish colored light. These cones contain three types of molecules that change shape with light of wavelengths in the red, green, or blue areas of the spectrum. You really see only red, green, and blue. The rest are combinations of these three colors.

So, we actually see only three pure colors among the infinite pure colors in nature. You knew your eyes weren’t perfect, but did you know they were this bad? At least we’ve been able to invent instruments that can detect the rest of the colors.


Mixing colors

Newton didn’t stop with this discovery. He wanted to know more about light. He’d seen that white light from the sun was made up of many colors and that you could separate them with a prism. Could that process somehow be reversed? Could the colors be mixed back together to make white light?


You probably know the answer already. Perhaps you’ve seen a wheel with all the colors of the rainbow printed on it that becomes white when set to spin. But that wheel was created after Newton answered the question.

Newton added a third prism to his experiment and passed a separate beam of light through it by making a second hole in the shutters. Then, he overlapped the two rainbows from the two prisms and formed white light.

Years later, when he was a famous scientist, Newton went back to this experiment and used a lens to converge the spectrum from a single prism into a spot. The spectrum disappeared into white light at that spot but, as it continued its path beyond that point, spread out and separated into its component colors.

Pitting Particles Against Waves

Newton thought that light was made up of particles. However, he accepted that light showed some aspects of wave behavior. He thought, for example, that the different colors of light had different wavelengths.

What’s the big deal about wave versus particle?

It turns out that the properties of waves and the properties of particles are exclusive. It’s like day and night, fast and slow, rich and poor. If you have one, you can’t have the other. You can’t have both particles and waves at the same time.

Exhibiting distinct behaviors


The best way to observe the properties of waves is with water waves, because you can see them. If you throw a pebble in a lake, you see the circular ripples spreading out. If you throw two pebbles, two sets of ripples spread out and, at places, run into each other. These ripples don’t bounce off each other — instead, they pass through. And when they overlap, there are areas on the surface of the water that are higher and regions where the water is flat. If you observe carefully, you can see a pattern.

Sound is also a wave. It’s formed by pushing molecules of air (or water or a solid) together. When you speak, your vocal cords vibrate and push the air molecules away. These molecules bump into neighbor molecules, and the whole thing spreads until the vibrations get to your friend’s ear and cause her ear membranes to vibrate. Then she can hear you.

If your friend is out in the hallway and you are in a room where you can’t see her, she still can hear you. Sound waves bend around corners. In fact, all waves bend around corners.

Particles, on the other hand, don’t have the same behavior. They bounce off each other when they collide, and they don’t bend around corners.

Believing that light doesn’t bend

Newton didn’t fully accept the idea that light was a wave because light doesn’t seem to bend around corners. You can see sharp shadows, which would seem to be evidence that light travels in straight lines.

However, even in Newton’s time, there was some evidence that light bends around corners. An Italian scientist named Francesco Maria Grimaldi had passed a light beam through two narrow slits, one behind the other. The beam then fell on a black surface. He noticed that the band of light on that surface was a tad wider than the slits, and he concluded (correctly) that the beam had been bent slightly at the edges of the slits. He named this phenomenon diffraction.

Newton knew of Grimaldi’s experiments but thought that the bending was due to the light particles bumping into the edges of the slit. Without further studies, both ways of looking at this phenomenon were equally valid. Because of Newton’s great standing in the scientific world, his view was accepted more widely.

Young: Showing that light is a wave

A century after Newton, in 1802, an English scientist by the name of Thomas Young improved Grimaldi’s experiments. Young passed a beam of light through a pinhole that he punched on a screen. This light spread out from the hole and passed through a set of two pinholes that he had punched side-by-side on a second screen. He used a third screen to observe the pattern of dark and bright regions that he had made.

Young knew very well that what he was seeing was telling him that light was a wave. The light behaved like water ripples that run into each other; it made similar patterns.

Creating coherent beams

Young’s interference experiment, as we call it today, was very clever. It turns out that you can’t get these interference patterns with a regular source of light. The reason is that the light beams from a regular source don’t vibrate in lockstep, and when they run into each other, they don’t form these patterns. You need two beams that vibrate in step — what we now call coherent beams — for the experiment to work.

How did Young manage to make coherent beams? The holes in the screens did it. The light beams from the two holes in the second screen had the same origin; they both came from the light passing through the first hole. Because Young placed the two holes in the second screen at equal distances from the first hole, the two light beams that came out of the holes in the second screen were coherent.

Calculating wavelengths

Young did more experiments, replacing the two pinholes in the second screen with slits (see Figure 7-6). The pattern of bright and dark regions became parallel bands. Using simple geometry, he calculated the wavelength of the light he used from the distances between the lines.

Figure 7-6: Young showed that light formed an inter-ference pattern, the signature of a wave.

Figure 7-6: Young showed that light formed an inter-ference pattern, the signature of a wave.

From his calculations, Young found out that the value for the wavelength of light was much smaller that Newton thought. The longest wavelength in the visible spectrum is the one for red light, which is less than one-thousandth of a millimeter. That’s why light casts sharp shadows and doesn’t appear to bend around corners. You need tiny objects, like Young’s pinholes, to detect the bending.


A Young prodigy

Young had been a child prodigy. When he was 4 years old, he read the Bible twice. He spoke eight languages by the time he was 14. He studied medicine at the universities of Edinburgh and Göttingen, where he graduated in 1796. He practiced medicine all his life but was not a very good doctor because of his poor bedside manner. He was more interested in science than in medicine and didn’t pay enough attention to his patients.

While in medical school, Young discovered how the lens of the eye changes shape when focusing at different distances. He later discovered that astigmatism was due to imperfections in the curvature of the cornea. From studying the eye, Young moved to the nature of light.


Meeting resistance

Young’s experiment is what we call a landmark experiment. It’s now repeated in schools around the world to demonstrate to students the wave nature of light.


His experiment showed, without any doubt, that light is a wave. The interference pattern that Young saw with his setup is the mark of a wave. You can’t get that pattern with particles bouncing off the edges of the pinhole. The particles would have to have a coordinated motion to be able to form such a symmetric pattern.

You’d think that with this irrefutable proof, the wave nature of light would become well-established right away. But it didn’t happen that way. Young’s experiment went against the teachings of the great Newton, and the English physicists were not going to have any of it.

Young tried to tell people that Newton himself was really not against the wave nature of light. Newton accepted the idea that the different colors of light had different wavelengths. But sometimes when people have strong beliefs, arguments are not enough to persuade them.

It wasn’t until 1818, when two French physicists came up with a complete wave theory based on mathematics, that the wave theory was finally accepted. It hasn’t been challenged since.

Oh, except that Einstein later said that light was made up of particles. More on that subject in Chapter 16.

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