19.4. The Electromagnetic Spectrum http://www.ck12.org
The Speed of Light
In 1675, thirty-three years after the death of Galileo, the Dutch astronomer Ole Roemer (1644-1710) made the first
successful estimate of the speed of light. He noticed that the period of Jupiter’s moon Io (revolving about Jupiter)
seemed to vary. He reasoned that when the Earth was farther (or closer) in its orbit from Jupiter, the light reflecting
off the surface of Io would take longer (or shorter) time to reach the Earth. Using Roemer’s data, the Dutch physicist
Christian Huygens calculated the speed of light as 2. 3 × 108 ms. Though this is more than 23% less than the accepted
value today, calculating it was a major achievement at the time. It was the first substantial evidence showing that the
speed of light was finite.
It was not until 1849 that the French physicist Armand H. L. Fizeau (1819-1896) devised an experiment which gave
the speed of light to within a little more than the 3% of the modern value.
Fizeau’s experiment relied upon a rotating gear. A beam of light was directed between the spaces of the gear’s teeth.
For example, the light passing through one opening traveled to a distant mirror and was reflected back through an
adjacent gear opening. Knowing the rotation rate of the gear and the distance the light traveled, the speed of light
could be determined.
In 1862, the French physicist Jean Foucault (1819-1868) improved upon Fizeau’s method by replacing the gear
with a rotating mirror (described below). His result for the speed of light came within less than one percent of the
present-day value.
About thirty years later, the American physicist Albert A. Michelson began a series of experiments using Foucault’s
method. Michelson used a rotating eight-sided mirror. Light was reflected off one of the mirrors to another mirror
far away. As the mirrors rotated, the reflected light returned. If the returning light fell upon one of the rotating
mirrors just right, it would further reflect into a narrow tube along which an observer could detect the light. Again,
knowing the rotation rate of the mirrors and the distance the light traveled to the distant reflecting mirror, the speed
of light could be calculated. Michelson’s final experiment in 1926 gave the speed of light (in a vacuum) to within of
the current accepted value ofc= 2. 997792458 × 108 ms.
Unless more precision is needed, we will use the value for the speed of light in a vacuum asc= 3. 00 × 108 ms.
Note that the symbolcis used to denote the speed of light.
As far as we know today, nothing travels faster than light through a vacuum. The speed of light plays a fundamental
role in physics.
Electromagnetic Waves
But whatislight?
A satisfactory answer to this question had to wait until the mid-19th century, despite the best efforts of Newton,
Huygens, Young, and others.
In our discussion of electromagnetic induction, we concentrated on the induced voltages and currents produced by
changing magnetic flux. We explained those phenomena using wires and magnets. It turns out, however, that neither
wires nor magnets are necessary in order to produce electromagnetic induction. The famous Scottish theoretical
physicist, James Clark Maxwell (1831-1879),Figure19.13, showed that electric and magnetic fields were the
fundamental constituents of electromagnetic induction.
We briefly state some of his conclusions:
- A changing magnetic field induces an electric field.
- A changing electric field induces a magnetic field.
- The magnitude of the induced field is proportional to the rate of change of the field.
- In all cases, the electric and magnetic fields are at right angles to one another, as shown inFigure19.14.