Q. 2

Consider Galileo’s method of measuring the speed of light using two lanterns. To get an accuracy of about 10%, the time taken by the experimenter in closing or opening the shutter should be about one tenth of the time taken by the light in going from one experimenter to the other. Assume that it takes 1/100 second for an experimenter to close or open the shutter. How far should the two experimenters be to get a 10% accuracy? What are the difficulties in having this separation?

Answer :

As we know,

Speed of light = 3 × 108 ms-1


According to the question,


To get an accuracy of about 10%, the time taken by the experimenter in closing or opening the shutter should be about one tenth (1/10) of the time taken by the light in going from one experimenter to the other.


Distance travelled by the light in 0.1s = 0.1s × 3 × 108 ms-1


=3×107 m


=3×104 km


Hence, difficulty in separation of that distance will be the curvature of the earth. As we know, the shape of the earth is an oblate spheroid, it has a curved surface. Because of the curvy property, light from one of the experimenters won’t reach the other.


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