¶ … 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.
Light Energy
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...
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