Light Energy illustrates the techniques employed by various scientists in determining the accurate value of the speed of light. This paper outlines all the methods employed by them and the grave difficulties they faced in acquiring a more accurate value. This paper also emphasizes on the importance of the value of the speed of light.
One of the fundamental concepts that is put forward for the students, who are studying about light is that it is a form of energy. Even though light shows properties like reflection and refraction, Huygens suggestion that light consists of waves in1680 was rejected by many scientists. This was because scientists then were not able to demonstrate the wave properties of light such as interference and diffraction. Even Newton considered light to constitute of small particles which he referred to as corpuscles. It was only later, in 1801, when Thomas Young observed the interference of light using a double slit arrangement that Huygens wave theory of light was revived. Through various experiments scientist were finally able to decipher the speed of light to be 299,792,458 m/s. An important point to note here is that this is the speed of light measured in a vacuum.
Scientists for many years faced great amount of difficulties in measuring the speed of light. This was primarily due to the fact that the speed of light varied in different mediums. An important question that pops into our mind at this point is that is their any connection in the difference in the speed of light with the refractive index of a medium? It is known that light moves much slower in water or glass as compared to a vacuum. "The ratio whereby light is slowed down is called the refractive index of that medium" (Botha 2002, The Speed Of Light).
Many years ago people had developed a concept that light travel instantaneously. "They thought so because after a military artillery fired at a large distance, they saw the flash immediately, but sound took a noticeable delay before you heard it" (Botha 2002, The Speed Of Light).
Galilio Galilei after putting forward several suggestions carried out experiments to measure the speed of light in 1667. In Galilio's experiment two people were made to stand a mile apart from each other. Both men were made to hold covered lanterns. When the first person uncovered his lanterns, the second person too would uncover his lantern the very moment he saw light from the first lantern. A third person was made to measure the time taken for the second person to uncover his lantern after seeing light from the first. This experiment was conducted a number of times but unfortunately for Galilio the speed of light could not be measured accurately. The only conclusion that Galilio came up with was that the speed of light was ten times faster than that of the sound.
After Galilio, came a Danish Astronomer named Ole Roomer. In 1676, Roomer made a symmetric examination of Io, one of the moons of Jupiter. Roomer discovered that as Io steadily revolved around Jupiter's orbit, Jupiter eclipsed Io at constant intervals. To his surprise he found that the eclipses lagged more and more behind the expected time for few months and then suddenly picked up the pace. In September 1676, Roomer correctly predicted that the eclipse on November 9 would be exactly 10 minutes behind schedule. He then told his colleageus at the Royal Observatory in Paris that,
As the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io, actually reflected sunlight, of course, took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth's orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter (Fowler
1996, The Speed Of Light).
After Galilio and Roomer, the next man to try his luck in measuring the speed of light was an astronomer named James Bradley who in 1728 sailed on the Thames with his friends. While sailing, he observed that even though the wind was unwavering, the little pennant on top of the mast altered its position each time the boat put about. He then visualized the boat as the earth in the orbit, the wind as some starlight reaching the boat from some remote star and reasoned the direction in which the boat was moving in would depend upon the direction of the starlight.
Bradley reasoned that the apparent direction of incoming starlight must vary in just this way, but the angular change would be a lot less dramatic. The earth's speed in orbit is about 18 miles per second, he knew from Roomer's work that light went at about 10,000 times that speed. That meant that the angular variation in apparent
incoming direction of starlight was about the magnitude of the small angle in a right-angled triangle with one side 10,000 times longer than the other, about one two-
hundredth of a degree. Notice this would have been just at the limits of Tycho's measurements, but the advent of the telescope, and general improvements in engineering, meant this small angle was quite accurately measurable by Bradley's time, and he found the velocity of light to be 185,000 miles per second, with an accuracy of about one percent (Fowler 1996, The Speed Of Light).
After Bradley came Louis Fizeau. His techniques relied upon Galilio's experiment but in a more practical way. He made use of a rotating wheel with hundred teeth and by showing a beam of light through them. He also made use of a mirror that would reflect the beam of light back through the same gap between the teeth of the wheel.
There were over a hundred teeth in the wheel. The wheel rotated at hundreds of times a second - therefor thousands of a second was easy to measure. Light was reflected from mirrors more than 5 miles apart. This also helped him making accurate measurements. By varying the speed of the wheel is was possible to determine at what speed the wheel was spinning too fast for the light to pass through the gap between the teeth and back through the same gap. Fizeau calculated the speed of light to be
313,300 Km/sec (Botha 2002, The Speed Of Light).
In 1926, after Fizeau's experiment, Foucault measured the speed of the light using a rotating mirror. The light from the mirror reflected back at an angle that was different from the angle with which it hit the mirror. Of course it was rather difficult to measure speed with the help of an angle but through the hard effort of 50 years and by increasing the accuracy of his employed technique, Foucault determined the speed of light to be 299, 796 Km/s.
Michelson measured the speed of light by employing the techniques of both Fizeau and Foucault. He made use of a very high quality mirror placed 2000 feet apart from the toothed wheel. He also made use of very high quality lenses to point the light on to the mirror. He finally measured the speed of light to be 186, 355 miles per second.
Following Michelson was Morley in 1887. He together with Michelson invented an interferometer that sent out two beams of light.
One beam of light was pointed along the direction of the earth's motion, the other light beam was pointed at a 90 degree angle relative to the first light beam. The beams were reflected by mirrors and focussed at a common point where they could interfere with each other. The light beam travelling along the direction of the earth's motion around
the sun was expected to travel at a different speed compared to the light beam travelling at 90 degrees compared to this light beam. They did not encounter any interference that suggested these two beams of light traveled at even slightly different speeds. They would have been able to measure a difference in speed if there was one.
They could not detect any difference in speed between these two light beams (Botha
2002, The Speed Of Light).
During the year of 1958, Froome by making use of a microwave interferometer and a Kerr cell shutter determined the speed of light to be 299, 792.5 Km/s.
To measure the correct the speed of light has always been a serious issue for many scientists because its value is not easily determined. For the earlier scientist, this task was even harder because they were not equipped with better apparatus that they could use in their experiments. With the passage of time, much advanced apparatus, techniques and scientific theories were discovered, which would help the future scientists with their experiments. The difficulties that Fizeau, Foucault, Michelson and Morley could have faced in…