The Speed of Light in Air and in Water (1959)

in this film we are going to measure the velocity of light then we are going to compare the speed of light in air and the speed of light in water light travels so fast that it was long thought to be instantaneous and even after it was believed that light travels at a finite speed it was centuries before men had devised equipment sufficiently precise to make a successful measurement actually it was over a century ago that fizo made the first such measurement and since that time many other scientists have made successively more accurate measurements of this most famous physical constant the veloc of light in air approximately 300,000 km/ to measure such extraordinary speeds special conditions are necessary on the one hand a measurement can be made using relatively simple timing devices if we employ extraordinarily long distances astronomical distances for example this is what rmer did in 1676 using the eclipses of the moons of Jupiter on the other hand we can use Simple convenient relatively short distances which can be accurately measured if we employ timing devices that are sufficiently delicate to measure very short time intervals millisecond microc or even small fractions of a microsc this precise and sensitive oscilloscope is just such a device and it forms a key element in the first of our two experiments with this electronic clock and this other equipment which you see grouped here we went outside and made a quite satisfactory measurement of the speed of light we went Outdoors because we wanted a good sized carry for our beam of light yet one that was manageable and we finally set up on a high school playground we wanted a light path which was 300 M long because we know that light travels 300 M in 1 microc but we intended to fold our light path once which meant that we had to set up where we had 150 M of straight Thro this is the scheme which we used our light source was located at the focus of this parabolic mirror it sent a beam of light in this direction to a plain mirror located at the far end of the field this mirror folded our path and reflected it back to this parabolic mirror here with a photo cell at its focus and the photo cell was connected to our timer now our aim was to measure the time of travel over a distance but what we actually measured was the difference in time of travel over two distances a short path and a long path we arranged to catch a fragment of our light beam on a small mirror here reflected across into another small mirror here and off that into our receiving Parabola this gave us both a short light path and a long path the light source placed in the focus of this parabolic mirror here was a spark gap pulsed by a 60 Cycle alternating current it was a first class light source for our purpose providing a rapid succession of very short light pulses the oscilloscope our electronic timer was arranged to trigger on the receipt of a light pulse put out by the spark gap and picked up by our photo cell the short path performed the double function of triggering the oscilloscope and providing us with a first blip on the cathode ray tube if uh this circle represents the face of the cathode ray tube then the first blip might look like this and the signal returning via the long path should then produce a second blip like this if the difference in distance between the short and long paths is 300 M as nearly as we can measure it then the time difference between our two blips should be almost exactly 1 microsc so we adjusted the duration of the sweep on the scope to be 2 microc on the face of our scope there is a scale if the total duration of the sweep is 2 micros then each major division should represent 2/10 of a microc and we would expect five divisions between our two blips to check that our oscilloscope was properly calibrated we used the standard signal generator with our plans complete and our equipment ready we went out to the playground we began by placing the distant plane mirror and then carefully measuring off 150 M from it the face of the plane mirror was vertical and we were careful to run our measurement out normal to its surface small pegs made it easier for us to double check our measurements we marked the 150 m point with a broad take we positioned the parabolic mirrors a short Way Beyond the measured distance as we had planned in the source mirror we fixed a clear 150 W bulb to help us with the alignment of our equipment we checked to see that the bulb was properly in the focus of the parabola and then we began the process of lining up the big mirrors the first step was to place a white marker on the line of sight between the source and the plain mirror then we adjusted the parabolic mirror until the beam was lined up on the marker we found that very small movements of the tripod legs shifted the beam through a considerable Arc we had to carry out a similar maneuver at 4 four or five distances each one further away to guide the beam of light both horizontally and vertically directly into the distant plane mirror a small adjustment on the distant mirror then brought the beam back into the receiving parabolic mirror if we were really getting our beam of light back here we ought to have a bright Trace show up on our translucent screen at the focus of the receiving parabola we placed the short PAAD mirrors carefully to ensure that the difference between the short and long paths was just 300 M we found that we could use quite small mirrors at this close range to produce a return image similar in intensity to the one coming back on the long path when we had both images coming in well we had carried our lining up as far as we could in daylight all that remained to do before dark was to sling the oscilloscope into position check our power connection and pick up our tools in the darkness the 150 W bulb created an effect like a search light because of the parabolic mirror we put the spark gap in its place though this was a relatively small light source the efficient focusing of the parabolic mirrors gave us a bluish trace on the screen at the receiving end which was sufficiently strong for our photo cell so we made our connections and switched on the oscilloscope with a long path blocked once more we adjusted our oscilloscope the short path was triggering the scope perfectly and giving us a good blip with a clearly defined Peak then we took away the baffle and sure enough the long path brought in a good strong signal as you'd expect this blip looked very much like the other so for our distance of 300 M the time measurement the separation of our two peaks was five divisions plus or minus perhaps one or two t0 of a division this was just about 1 microc as we had anticipated the principal limiting Factor on the accuracy and precision of our measurement of the velocity of light was the resolution of the signals on the cathode ray Tu and of course the accuracy also depended on the exactness of our distance measurement and on the calibration of our electronic clock but you have seen that we were able with relatively modest equipment to make a fair stab at a measurement of the velocity of light in air now it remains for us to discover whether light travels at a different speed in water and if so what difference in speed there is the first measurements of the velocity of light in two different Media made over a century ago caused scientists to alter radically their theories on the nature of light so it will be worth our while to make this measurement for ourselves in this experiment the basic pattern of the light travel is from a light source here to one corner of our work area where we have a very small plane mirror mounted on a shaft which we can run rotate this movable mirror reflects the light toward the opposite corner of our work area where it is caught by each of two plane mirrors one of those plane mirrors is submerged in water at the end of a trough the other stands in the air beside the trough these mirrors reflect the beams they catch back to the small movable mirror and in turn this rotating mirror reflects those light beams back to the source the small mirror which we can cause to revolve is mounted here at the top of this black casing now here you can see the two different light paths the one for air and the one for water the air path leads to the distant mirror on the right the water path is to the left traveling down the trough it enters the water trough by means of mirrors arranged as a sort of periscope then it travels through the distilled water to a plain mirror at the other end back to the Periscope and so back to the revolving mirror the part of the paths between the source and the revolving mirror and back are of course common to both light paths the light source for our experiment is a carbon Arc which delivers a strong light focused on the revolving mirror between the light source and the revolving mirror we have a mask which cuts off all but a narrow slit of intense light when this narrow beam of light strikes the revolving mirror it is reflected away as a pencil beam which scans through an arc about the Pivot Point as the mirror revolves it throws the reflection much like a search light scanning across the sky now the only fragments of this scanning beam which concern us are those fragments which are returned as the pencil beam scans across the mirrors at the far end of the path the distant mirrors reflect the fragments or pulses back to the revolving mirror and the revolving mirror in turn directs them back toward the source now for the moment let's consider just the air path during the time that the pulses are in transit to the distant mirror and back the revolving mirror will turn through a small angle and thus the return beam reflected from the revolving mirror into the source will be displaced through a small Angle an angle dependent on the time of Transit and on the speed of rotation of the mirror now we know that light travels at a tremendous speed so it is clear that if we are going to get a deflection of the return beam which is large enough to be observed we will have to be prepared to rotate our mirror at a very high speed to accomplish this we have mounted our mirror on the shaft of a motor which can be revved up to 500 revolutions per second that's 30,000 revolutions per minute this is our variable Transformer here I'll turn up the speed of the motor while you listen to the pitch of the sound sounds like a sewing machine like a vacuum cleaner like a power sander but listen to how much higher I can turn up the speed [Applause] yet so you see we have a very high speed available we shall see whether it is enough to shift the return IM of the slit away from its Point of Departure now it's obvious that we need some way of observing our return beam to see whether it is moved or not to solve this problem we have introduced a half silvered mirror here at a 45° angle about half the light from the source will pass through it on its way to the rotating mirror the other half being dissipated off to this side on its return course half the light will pass through to the source but but the other half will be reflected onto a small scale in our viewing device in this pattern of light pads we have had to introduce lenses to focus and control our light intensity a card over this lens will block the water path temporarily so that to start with we will be using the air path only now we ready to make our first observation I will start the rotating mirror note where the light image appears when our mirror is revolving slowly even speeds up to 80 or 100 revolutions per second produce no apparent displacement of the image but watch what happens as I increase the speed still further notice how the image is being displaced to the right at a speed of 500 revolutions per second it has moved about 2 and 1 half divisions on our scale with full knowledge of the distances angles speed of rotation of the mirror we could calibrate our scale so as to read off directly the time of travel as we did in the other experiment and thus calculate the speed of light in air but our concern here is to compare the speed of light in two different media air and water now we've arranged our viewing device so that the pencil which is traveled entirely through the air will appear primarily above the horizont line on the scale now let's remove the obstruction on the water pad so that we can observe both pads at once the pencil which is traveled through the water will appear primarily below the horizontal line here is the way the water pencil will look and here is the air pencil again water air now you can see both pencils at once the mirror is revolving slowly and the traces are in their zero position for our first comparison we have set the distant mirrors at exactly the same distance from the rotating mirror now will the two traces on our scale keep step with each other as I increase the speed of rotation that would imply that the time of travel over the two paths is the same let's watch the scale what do you see the air pencil is assumed the same position as before but the water pencil that's the lower one you remember has moved out past it the relative position of these two traces indicates that the pencil which traveled through the water took a good deal longer to return than the other the speed of light then in water is slower than it is in air to determine how much slower we have gradually lengthened the air path until the two traces coincide now with this longer air path let's observe the scale again as I increase the speed of rotation the two traces keep stepping all the way they're both there air and water with this longer air path then the time of travel is the same now we've measured the relative length of these two paths the distance the light travels up to this point and back through the air is of course common to both paths but from this point onward the length of the round trip path in the water is 18.8 M and in the air 25.5 m a ratio of about 3 to 4 so you can see that light travels considerably more slowly in water than it does in air indeed just about 3/4 as fast in your work you have found that the index of refraction for light traveling from air into water is 4/3 is there any connection between the index of refraction and our measurement or is the agreement just a coincidence