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HC Verma for Class 11 Physics Chapter 21:Speed of the Light

Chapter 21 focuses on different experiments to verify the speed of light. Foucault and Michael's experiment are two important topics to be considered. Fizeau, in 1849, performed the first terrestrial measurement experiment to determine the speed of light. He projected a pulsed beam of light on a mirror placed at a distance. Considering the speed of rotation and the number of teeth on the toothed wheel, the distance between the mirror and the light source, he could calculate the speed of light to be 315,000 km/s.

Fizeau's experiment was later modified by Foucault, who replaced the toothed wheel with a rotating mirror. With this new arrangement, Foucault determined the speed of light to be 298,000 km/s, much closer to today's accepted value. In the Foucault method, a light source is used to focus a beam on a rotating mirror. The rotating mirror then reflects this light onto a fixed mirror at an angle. The fixed mirror is aligned in such a way that it faces the reflected light beam perpendicularly. Thus, the light beam is reflected directly back onto the rotating mirror from where it was first reflected.

While the light travelled between the two mirrors, the rotating mirror had changed its orientation to the beam of light. Therefore, the returning beam of light is reflected at a different angle. Thus, we can imply that the difference in the angle of the light source to the rotating mirror and that of the rotating mirror is related to the time taken by light to travel the distance between the mirrors.

Further, Michaelson tried to determine the speed of light with a different method. He devised a method of measuring the speed of light directly by finding how long it took to move a measured distance. The idea behind this experiment was to find out the speed of light in different directions. This data would be useful in measuring the speed of ether relative to Earth, thus establishing its existence.




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