LIGHT, FRANCIS

born c. 1740, , Suffolk, Eng. died Oct. 21, 1794, Penang Island [now in Malaysia] British naval officer who was responsible for acquiring Penang (Pinang) Island in the Strait of Malacca as a British naval base. Light served in the Royal Navy from 1759 until 1763. In command of a merchant ship, he went in 1771 to the northern Malay state of Kedah, where he won the confidence of the sultan, Mohammed. About that time England, at war with France, was looking for a suitable naval outpost along the Malay Peninsula. By March 1786 the East India Company, apparently at Light's urging, settled on Penang as the site. Light conducted the negotiations with Mohammed's son, Sultan Abdullah of Kedah, who was threatened by the powerful states of Siam (Thailand) and Myanmar (Burma). Abdullah agreed to English occupation in exchange for support against other Southeast Asian powers. Penang was annexed on Aug. 11, 1786, but the British allowed Siam to take over control of Kedah early in the 19th century. Light governed the settlement, which was declared a free port. His generous land grants and encouragement of trade attracted a number of immigrants, particularly Chinese, and the area soon prospered. Interference and diffraction phenomena Interference Quasi-monochromatic waves Figure 13: Damped waves. (A) Amplitude, x(z), as a function of distance, z. A perfectly monochromatic wave, represented by equation (1), has constant amplitude and is not limited in space or in time. Sources of light (other than lasers) emit waves the amplitude of which varies with time. For example, a single undisturbed atom emits a damped wave (Figure 13A). Under favourable conditions the damping is so weak that 107 waves are emitted before the amplitude has fallen to half its initial value and the change of amplitude is not significant over a distance of several thousand wavelengths. Wave trains of this type are said to be quasi-monochromatic. Superposition of these waves gives interference when the path difference is not too large. Photometric summation Figure 4: Interference fringes obtained in Young's experiment (see text). Figure 4, curve A, shows the way in which the intensity of light varies from place to place when two monochromatic or quasi-monochromatic waves overlap. The intensity at a point in the region where the waves overlap may be expressed as the sum of two terms: (1) the sum of the intensities of each wave acting alone (2I0 if each alone would give intensity I0); (2) a term representing the interference of the waves. The second term varies from point to point along the direction of propagation between the values -2I0 and +2I0. Thus the total intensity varies from 4I0 (i.e., twice the intensity sum) to zero. Now, when a large number of waves from different sources cross a certain space, the fringes caused by the interference of each pair of waves have their maxima in different places and the overall result is that, at any point, the interference terms are positive nearly as often as they are negative and their total sum is nearly zero. In this case the resultant intensity at any point caused by a number of sources is just equal to the sum of the intensities (at that point) of each source acting alone. This is the law of photometric summation and is used by illumination engineers in calculating the illumination on a surface that receives light from various sources. Interference fringes are obtained only when experimental conditions are such that the interference fringes caused by light emitted from different atoms all have their maxima in the same places (or near to the same places). The interference term then becomes a significant fraction of the summation term. This may be achieved either (1) by using two secondary sources (such as the two slits used in Young's interference experiment), which are both derived from the same primary source, or (2) by using a laser in which the source atoms are stimulated in such a way that the phase relations between them remain constant during the period of observation. Polarization and electromagnetic theory Polarized light Interaction of plane-polarized beams Figure 3: Young's experiment (see text). Fresnel and Arago, using an apparatus based on Young's experiment (Figure 3), investigated the conditions under which two beams of plane polarized light may produce interference fringes. They found that: (1) two beams polarized in mutually perpendicular planes never yield fringes; (2) two beams polarized in the same plane interfere and produce fringes, under the same conditions as two similar beams of unpolarized light, provided that they are derived from the same beam of polarized light or from the same component of a beam of unpolarized light; (3) two beams of polarized light, derived from perpendicular components of the same beam of unpolarized light and afterwards rotated into the same plane (e.g., by using some device such as an optically active plate) do not interfere under any conditions. Result (1) is to be expected because two displacements in perpendicular planes cannot annul one another, and result (2) is also easily understood. Result (3) shows that mutually perpendicular components of unpolarized light in a beam are non-coherent. Their phase difference varies in time in an irregular way. Unpolarized light has a randomness, or lack of order, as compared with polarized light (implying an entropy difference). This order (or lack of order), rather than the azimuthal property, is the most fundamental difference between polarized and unpolarized light. Perfectly monochromatic light is perfectly coherent and completely polarized. Superposition of polarized beams Figure 18: Progression of elliptically polarized wave (see text). Figure 18: Progression of elliptically polarized wave (see text). Two coherent beams of plane polarized light may be thought of as propagated in the Oz direction, one with its vector along Ox and the other with the electric vector along Oy; i.e., the two vibrations are at right angles to each other as well as to the direction of propagation (Figure 18). If the beams have amplitudes ax and ay and phases ex and ey, then, in general, the resultant vibration (R1, R2, and R3) may be represented in magnitude and polarization by a vector, or arrow, the tail of which touches the axis of propagation Oz while the point moves round the ellipse (Figure 18). It goes round once when the phase angle f (see equation ) changes by 2pi.e., at any given place when t changes by y or for any one time when z changes by l. The beam is said to be elliptically polarized. If the phase difference is p/2, then the axes of the ellipse are equal to ax and ay and are along Ox and Oy. Elliptically polarized light may be regarded as the most general type of polarized light. If the amplitudes of the two waves are equal, ax = ay, and the phase difference is still p/2, the ellipse becomes a circle and the light is said to be circularly polarized. If the phase difference exy is not equal to p/2, the resultant is still elliptically polarized light, but the axes of the ellipse no longer coincide with the axes of coordinates. If the phase difference exy = 0 or p, the ellipse shrinks to a straight line and the light is said to be plane-polarized. If the representative vector, when viewed by an observer who receives the light, rotates in a clockwise direction, the light is said to be right-handed (or positive) elliptically polarized light. The opposite sense of rotation corresponds to left-handed (or negative) elliptically polarized light. In the above analysis, elliptically polarized light is regarded as the resultant of two beams plane-polarized in perpendicular planes. Conversely, it is possible to regard plane-polarized light as the resultant of two beams of elliptically (or circularly) polarized light of the same wavelength, provided that the ellipses are similar in orientation and eccentricity, but one beam is right-handed and the other left-handed.