Chapter 16, Problem 76A

To determine

To describe: An essay that demonstrates the history of human understanding of the speed of light including significant individuals and the contribution that each individual made.

Explanation of Solution

Introduction:

Many people gave many theories related to the concept of speed of light. The speed of light is calculated $c=3\times {10}^8\text{m/s}$ . Many scientists gave many values but all those were incorrect and rejected. Only the above-mentioned value is considered as the accepted value for the speed of light.

The contribution done by different scientist to determine the speed of light is given below,

Galileo: In the early 17^th-century, scientists believe that the speed of light is of infinite value and during this period it was Galileo who proposed that the speed of light can be measured and it is a finite value.To prove this,he experimented with the help of his assistant.

In the part of the experiment,Galileo and his assistant stood on a distant hill with lanterns. He knew the distance between them and he needs to measure the time taken by the light to travel from him to his assistants.With the distance and his time values,he thought of determining the speed of light. ButGalileo’s experiment did not succeed as he failed to count the seconds accurately that is the time taken by the light to travel between the hills was a tiny fraction of the second and measuring such a minute in number in those time without any proper instrument is not possible.Finally,Galileo concluded that the speed of light may be at least ten times faster than sound.

Ole Roemer: He was a Danish astronomer of the 17^th century.During the observation of Jupiter’s moons, he observes that the orbit of the Jupiter’s moons speeds up when they come close to Earth and slow down when Earth was at a farther distance from Jupiter. From these observations,he concluded that the things in the orbital of the moons are due to the apparent change which is caused because of the extra time taken by the light to travel from Jupiter to the Earth when they are far away from each other. He made use of the diameter of the Earth’s orbital and finally concluded that the speed of light is 200,00km/s.

James Bradley: He was an English physicist of the 18^th century.James used the concept of stellar aberration to determine the speed of light.Stellar Aberration causes a variation in the apparent location of the stars as a result of the revolution of the earth around the Sun. The ratio of the speed of the earth while revolving around the sun and speed of the lights gives the stellar aberration. James had an idea of the value of the speed of the earth revolving around the Sun and he could also measure the stellar angle,and through these two values he could able to estimate the speed of light in a vacuum is 301,000km/s.

Louis Fizeau: He was a French physicist. Fizeau used a toothed wheel to determine the speed of light. The incident light between the teeth of the rotating wheel and placed a mirror at a certain distance through which the reflected beam from the mirror travels back through the same opening between the teeth of the wheel. From this experiment, he could measure the distance the light traveled and the time taken by the light to travel. The ratio of these values gave the speed of light as 313,300km/s.

Leon Foucault:He was a French physicist.Foucault adopted the same method as that of used by Fizeau but used two mirrors instead of one.Out of the two mirrors, he set one for rotating and other was fixed. The light is made incident on the rotating mirror which got reflected on the fixed mirror and again it bounced back to the rotating mirror.From this rotating mirror, the light is bounced back on the fixed mirror with an angle that is different from the angle made at the first time. Foucault measured the angle and through this value, he could measure the speed of light as $299,796\text{ km/s}$ .

Albert A Michelson: Michelson used the modified form of Foucault’s rotating mirror and he could measure the speed of light accurately with his eight-endmirror.The light is made incident on one of the sides of the rotating mirror which got reflected on the fixed mirror placed on a mountain top and again it bounced back on the rotating mirror.If the rotating mirror rotates exactly 1/8^th of a turn, then the side will take the correct position to reflect the light. From this experiment, he determines the speed of light as $2.99\times {10}^8\text{ km/s}.$