NRLF B M 253 Sflfi LIBRARY UNIVERSITY OF CALIFORNIA. Class 0itmal of (irforti Ettikrsiig |mtior mitfif Clitb JUNE, 1906 The Pressure of Light BEING AN ABSTRACT OF THE THIRTEENTH ROBERT BOYLE LECTURE DELIVERED BEFORE THE OXFORD UNIVERSITY JUNIOR SCIENTIFIC CLUB On May jo, BY PROFESSOR J. H. POYNTING, F.R.S. OF THE UNIVERSITY ' F LONDON: HENRY FROWDE, AMEN CORNER, E.G. EDINBURGH: 12 FREDERICK STREET. GLASGOW: 104 WEST GEORGE STREET OXFORD: 116 HIGH STREET NEW YORK : 91 & 93 FIFTH AVENUE TORONTO: 25-27 RICHMOND STREET WEST 1907 Price One Shilling net The Pressure of Light BEING AN ABSTRACT OF THE THIRTEENTH ROBERT BOYLE LECTURE DELIVERED BEFORE THE OXFORD UNIVERSITY JUNIOR SCIENTIFIC CLUB On May jo, 7906 BY PROFESSOR J. H. POYNTING, F.R.S. LONDON: HENRY FROWDE, AMEN CORNER, E.G. EDINBURGH : 12 FREDERICK STREET. GLASGOW : 104 WEST GEORGE STREET OXFORD: 116 HIGH STREET NEW YORK : 91 & 3 FIFTH AVENUE TORONTO: 25-27 RICHMOND STREET WEST 1907 e THE PRESSURE OF LIGHT (Abstract) THE pressure which light exerts against any surface on which it falls, and which it equally exerts against the source from which it starts, would have received a far easier explanation a century ago than it does to-day. Then, light was supposed to consist of corpuscles shot out from the source. Each corpuscle, a lecturer would have said, was like a shot from minute artillery giving a kick back to the gun firing it, carrying momentum with it through space, and delivering the momentum up as pressure to the surface which it bombarded. This appeared so obvious that the pressure effect was often looked for in the eighteenth century. Results were obtained sometimes as a pressure, sometimes as a pull ; but they were really no doubt due to air currents or to radiometer action. Had these early observers known the mechanical equivalent of heat, they would have been able to calculate the pressure to be expected, and though a calculation on their corpuscular hypothesis would have given double the true value, they would have realized that it was far beyond their power to detect it with the experimental means used. When the corpuscular theory gave place to the trans- verse-wave theory, the idea of pressure was t lost sight of, and it was only revived thirty-three years ago, when Maxwell showed that light pressure was a direct con- sequence of his Electromagnetic Theory ; that electro- 1 B2920 4 THE PRESSURE OF LIGHT magnetic waves would carry momentum just as much as corpuscles would carry it. On. this theory light consists of a train of waves of electric displacement, accompanied by magnetic in- duction at right angles, both being at right angles to the direction of propagation. We may symbolize the waves by tubes of electric force, and tubes of magnetic force rushing through space. These tubes tend to contract along their length, and to push out sideways somewhat as a bundle of stretched india-rubber tubes would tend to contract lengthwise, and push against each other sideways. Thus there is a side pressure against each other, and since both kinds of tube are perpendicular to the direction of propagation, they will strike sideways against any surface on which the light falls, and will exert their sideways pressure on that surface. Though Maxwell reached the result by his Electro- magnetic Hypothesis, we now know that the pressure must exist whatever wave theory we adopt, if we assume that the energy in a cubic centimetre of a wave train of given amplitude varies inversely as the square of the wave-length. Let us first suppose that a surface emitting light is at rest. It is sending out waves of a certain amplitude and of a certain length, we may suppose, depending on its temperature. Now let the surface move forward in the direction of propagation. We may reasonably as- sume that the amplitude of the waves will be as before, but by the well-known Doppler effect their length will be less. It can be shown that the energy in the waves emitted will be greater. This extra energy implies that work has to be done in moving the light-giving surface THE PRESSURE OF LIGHT 5 forward, and therefore a pressure is exerted by the waves against which this work is done. The value found for the pressure shows that when the surface is at rest, it is equal to the energy in one cubic centimetre of the light emitted, and that it is rather greater when the surface is moving forward, rather less when it is moving backward. But the pressure back against the surface means that momentum in a forward direction is being poured out from the surface, and that it is travelling with the waves. This momentum is delivered up to any surface on which the waves fall. If the receiving surface is at rest, it will experience a pressure equal to the energy per c. c. in the incident beam. But if it is moving forward towards the source, it will receive more waves per second than if at rest, and so will receive more momentum and experience more pressure. If it is moving away from the source, it will receive fewer waves per second and less momentum, and so will experience less pressure. The pressure is exceedingly minute in any case. In full sunlight a surface at the earth experiences a pressure only of the order of 5 mgm. on 10 square metres. The whole earth experiences from sunlight only a pressure of about 75,000 tons. Even against the surface of the sun, the back pressure is only about ii mgm. per sq. cm. The first experiments to show this pressure of light were published by Lebedew six years ago. He sus- pended a small disc in a very high vacuum, and allowed a beam of light to fall on the disc. He found that the disc was repelled. Simultaneously Nichols and Hull were working in a similar manner, and they showed that the pressure observed was, as nearly as could be 6 THE PRESSURE OF LIGHT measured, equal to the energy in i c. c. of the beam. They succeeded in showing the equality within i / . Thus a beam of light receives momentum from its source, carries the momentum through space, and delivers it up as pressure to any surface on which it falls. Some experiments made by Dr. Barlow and the lecturer 1 bring out this idea that light is to be regarded as a stream of momentum. If a beam falls obliquely on an absorbing surface, it should exert both a normal pressure and a tangential stress on the absorbing surface. In the first experiment two small discs were fixed at the ends of a small horizontal rod suspended by a quartz fibre attached to the middle of the rod, the apparatus being in an exhausted vessel. The discs were vertical, and perpendicular to the rod, and one disc was blackened. When a horizontal beam of light was directed at 45 on to the blackened disc, the tangential stress drove that disc round away from the source. In this form of ex- periment it is difficult to eliminate disturbances. A better form of the experiment consists in suspending a single horizontal blackened disc by a quartz fibre from its centre. A beam of light is then directed on to a small part of the disc between the centre and the edge, and at 45 to the horizontal, so that the tangential stress tends to turn the disc round. This beam is then re- placed by an equal beam which comes on to the same part of the disc, but at 45 on the other side of the normal . to the disc. The tangential stress is now reversed. But the disturbances due to heating of the disc are the same as before. The difference between the two displacements is twice the effect of the tangential stress of either beam. 1 See Nature, Nov. 22, 1906, for a fuller description of these experiments. THE PRESSURE OF LIGHT 7 In another experiment two prisms were fixed with their refracting edges vertical, and turned in opposite ways at the ends of a short torsion arm. A beam of light was sent through one prism, so that it passed to the other prism in a line parallel to the torsion arm, and finally emerged from the second prism parallel to its original direction. The stream of momentum was thus shifted by the refraction parallel to itself, and this implied a couple acting on the light with an equal and opposite couple acting on the prisms. They turned round, as was expected, under this couple. On the surface of the earth the pressure of light has negligible results, for the disturbing effect of the atmo- sphere gives it no free play. But in the Solar system, where it is continuous in its action and has free play for ages, it may have quite appreciable effects upon small bodies. On the larger j bodies its effect is too minute for observation. Thus, sunlight presses on the earth with 75,000 tons in all. This virtually lessens the sun's gravitation pull, but only by one in 40 billion. If, however, we diminish the size of the body receiving the radiation, the ratio of light pressure to gravitation pull increases as the linear dimensions decrease. If then a sphere had the same density as the earth, and a diameter a forty-billionth of the earth's diameter, the light pressure on it would equal the gravitation pull, if the law of diminution held good down to such a minute body. But for such a minute body about a wave-length of violet light in diameter diffraction would come in, and we shall not consider bodies so minute as this. The effect of diminution of size applies to the radiator also ; and if we imagined the size of the sun diminished instead, its gravitation pull would bear a diminished 8 THE PRESSURE OF LIGHT ratio to its light pressure. The radiation depends on the temperature, and it can be shown that two bodies of the temperature and density of the sun would press each other apart by their mutual radiation, as much as they would pull together by their gravitation, if they were each 40 metres in diameter. If they were of the density of the earth, and of the earth's temperature, say 20 C., there would be a balance between radiation pres- sure and gravitation pull when they were about 2j cm., say i inch in diameter. We may probably have to consider radiation pressure in dealing with the mutual actions of a swarm of mete- orites if they consist of or contain particles of such a size. When far out in space, and cold, their gravitation pull would be greater than their radiation push, and they would tend to draw together. As they approached the sun they would get hotter, and the radiation push would increase. At the earth's distance they would, if about an inch in diameter and five times as dense as water, neither attract nor repel each other. As they approached still nearer, they would get hotter, and radiation push would be greater than gravitation pull. So they would tend to move apart. If Saturn's rings consist of small particles warmed by the planet, they too may be of such size that radiation pressure is comparable with their mutual gravitation. There is no doubt that small bodies exist in the Solar system. We have evidence for their existence on any starlight night, when they perish in the air as shooting stars. Let us trace the effect on their motion of the radiation pressures. Imagine a small black particle T ^ou inch in diameter THE PRESSURE OF LIGHT 9 of the earth's mean density, 5^, to be circling round the sun at the earth's distance. The pressure of sunlight on it is T ^ of the gravitation pull. Then its gravitation is virtually lessened by i in 100, and it will require a less velocity than the earth to keep it in its circle. Its year will be about 367 instead of 365 \ days. Then it is heated by the sun, and giving out radiation on all sides. This radiation presses on its surface as it goes out. But the pressure is slightly greater on the front side than in the rear, for we have seen that forward motion increases and backward motion decreases the pressure against a source. Hence there is always excess of pressure in front, tending to resist the motion. As a result the particle moves spirally in towards the sun, and in one year about 800 miles. As it moves in and gets nearer the sun, it gets hotter, and the resist- ing effect increases. Such a particle will probably reach the sun itself from the distance of the earth in some time between 10,000 and 100,000 years. If we could establish observatories out in space to watch the motion of such small bodies from year to year, we could use our observations to measure the variation in the output of energy from the sun from year to year. Light pressure, then, though one of Nature's minutest forces, may yet have very great effects when acting on minute bodies. Such minute bodies appear to abound in our system, forming the dust of space. This dust the sun is ever sweeping in towards himself. The particles may have a life of thousands, or even of millions of years, but sooner or later, if they are small enough, they must end in the sun. X 53 f UNIVERSITY I \< OXFORD: HORACE HART PRINTER TO THE UNIVERSITY at* UNIVEESITY OF CALIFORNIA LIBRARY, BERKELEY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW Books not returned on time are subject to a fine of 50c per volume after the third day overdue, increasing to ?$1 00 peTvolume after the sixth day.. Books not in demand may be renewed if application is made before expiration of loan period. NOV MAY 5 1925 NOV 1 IS 1931 AU6 23 1946 wOctSICf SOctSlLU **** *, i b/ s 15m-4,'24 liiiiiiiiiP 1