Light Travels At 196,000 Miles a Second!

Sun (Auckland), Volume II, Issue 460, 15 September 1928, Page 26

Light Travels At 196,000 Miles a Second!

(WRITTEN for THE SUN by C. M. GRAY.)

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[ENTISTS tell us that light travels at a velocity of 186,000 miles a second. And it may be so for all we can say to the contrary. But

in these days of enlightenment the man in the street, with his more highly-developed critical faculty asks for more proof than a mere statement. The actual process of measurement of the speed of light, the various means devised, and the comparative results, still remain among the most interesting even in this time of interesting discoveries. The actual speed of light was, as is generally known, found from observations made in the discrepancies between the calculated and actual times of eclipse of Jupiter’s satellites—this discrepancy appearing greater or smaller as we were further from or nearer to this planet. Thus it was found that it took eight minutes of time for light to travel from the sun to the earth, a distance of approximately 90,000,000 miles. This gives the velocity of light as approximately 196,000 miles a second. The possibility of calculating the velocity of a beam of light by direct measurement, however, appeared remote until a scheme was devised by a Frenchman ( named Fizeau in 1849, and in conjunction with Foucault in 1862, some amazingly successful results were obtained. The outline of bis method is as follows: A beam of light was passed through a narrow slit into a darkened room and allowed to fall on to the surface of a revolving mirror which we shall call A. The beam was then reflected to a curved mirror B, the arc of this mirror having its centre at A. From B the.light was reflected back to A and thence along the line of the initial

beam to a diagonal mirror of clear glass, which turned the beam at right angles into the lens of an observing eye-piece. If the passage of light from A to B and back to A were instantaneous, then, at all speeds tji the revolving mirror, the spot of light indicated on the lens of the eyepiece would take up the same position. If, on the other hand, the passage of light from A to B and back again occupied a definite time, the revolving mirror would, during that time, move through a part of its revolution before picking up the reflected beam and the indicated spot on the lens would show a slight displacement from its original position. Hence, from this displacement, knowing the length of travel of the beam and the speed of the mirror, could be calculated the actual speed of light. In Foucault’s experiment the length of beam between the revolving mirror and the curved mirror was only 13j.ft and it was necessary to give the revolving mirror a speed of 600 revs, a second to show a displacement of 1-100 of an inch in the eye-piece. By increasing the speed to 800 revs, a second, a displacement of 12-1,000 of an inch was indicated and by means of suitable magnification, this distance was measured with extreme accuracy. The speed calculated from this ex-

periment was 185,157 miles a second. Obviously small experimental errors in measurement, such as the distances between the mirrors and the speed of the revolving mirror, would be enormously magnified in obtaining the final result and in an attempt to reduce these sources of error, Professor Newcombe, of Washington, used a distance between the concave and revolving mirrors of some 21 miles with a mirror speed of 230 revs, a second. His calculation showed the speed of light to be 186,327 miles a second, indicating that Foucault’s error, working under unfavourable circumstances, was slightly more than half of one per cent. —a tribute to that experimentist’s accuracy and skill. Professor Newcombe states the accuracy of his experiment to be 1-10,000 and adds that by using a length of beam of 20 to 30 miles, such as could be obtained in the clear air of the Rocky Mountains, a much closer figure to perfect accuracy could be obtained. So reliable is this apparently simple method of measurement considered, that its result is used in the measurement of. distance between the earth and the sun—this distance being the unit of length used in astronomical calculations.