Page:The New International Encyclopædia 1st ed. v. 12.djvu/273

* LIGHT. in any one medium is constant (but varies with the medium) ; therefore for any such train of waves — is a constant and may be written n "z as before, the index of refraction of the second medium with reference to the first. Thus it is seen that the incident ray CC becomes C'C" by refraction, that they lie in a plane which includes tile perpendicular to the surface at C", and that their angles of incidence and refraction are given by the formula -= n, where n is a constant for any one train of waves. Thus n, and there- fore the velocity of ether-waves, may be found to vary both for ditl'erent colors in one medium and for the same color in different media. (Since it has been shown that in ordinaiy matter n in- creases as the color is changed from red to yellow, to green, to blue, etc., it is evident that the velocity of those waves which produce the sen- sation red — "red waves' — is greater in ordinary matter than is the velocity of "green waves,' etc. For if the first medium is the pure ether and tiie second air (or glass, or water), r, remains con- stant for all waves, and if n increases it must be because r, decreases. Double refraction is at once explained if the assumption is made that, whereas a point-source in an isotropic medium produces a spherical wavepnint. in media which have different prop- erties in different direetion.s a point-source pro- duces a more complicated wave-front. To ex- plain the simpler case of double refraction, that where one ray obeys the ordinary laws of re- fraction, Huygens assumed that the wave-front was a combination of a sphere and an ellipsoid of i-evolution. the axis of revolution being a diameter of the sphere. In bodies of this nature there is always one direction in which they be- have in all respects like ordinary transparent bodies. This is called the 'optic axis,' and it is evidently in the direction of the axis of revolu- tion of the ellipsoid of the wave-front. (It is not a line, but a direction : and so all lines drawn in this direction within the body are optic axes.) These bodies are therefore called 'uniaxial.' Let such a body have a face cut and polished making an angle 9 with the optic axis, and let the paper make a section perpendicular to this face and including the optic axis. If a plane wave- 24" LIGHT. stant the wave-fronts in the uniaxial body may be easily seen to be phines which pass through the line perpendicular to the paper at C and are tangent to the sphere and to the ellipsoid. The disturbance from A has tiius reached the points of tangcncy O and E, AO and AE are called the ordinary and the c.traordinary rays; and it is evident from geometrj- that the latter ray does not in general obey either of the laws of ordinary refraction. Two lines drawn perpen- dicular to the wave-fronts, e.g. B'O' and U'E', are called the wave 'normals;' and it is seen that in general the extraordinary ray docs not have the same direction as the extraordinary wave- normal, i.e. the ray is advancing in a direction different from the direction of advance of the wave-front. In other douljly refracting bodies there are two optic axes; such are called "biaxial.' Fresnel explains their optical properties by as- suming a peculiar kind of wave-surface, which is too complicated for description here. The wave-surface of Huygens for uniaxial bodies may be regarded as definitely established by experiment ; while all that can be said in regard to Fresnel's wave-surface for biaxial bodies is that it is most probably correct. Interferexce axd DiFFRACTiox. The optical phenomena whose discovery established the fact that light is due to wave-motion are those of 'interference' (q.v. ) so called. The simplest illus- tration of these is due to Thomas Young and is called after him. Three opaque screens ai"e ar- ranged in order ; in the first there is a narrow slit Fig. 17. front ABC is incident on this face, the dis- turbance at A in the uniaxial body sends out its dnulile wave-front, the section of which — a circle and an ellipse — is shown : and while the disturb- ance goes from C to C in the air let the double wave-front advance as shown. Then at this in- FlG. 18. at O, illuminated with some homogeneous light, e.g. a flame burning sodium; in the second there are two slits. O, and 0,. close together parallel to the first and at equal distances from it; the waves spreading out from O illuminate the slits Oi and 0- so that they in turn become sources of Itco identical trains of waves — if the source at flickers or changes in any way, both the second- ary sources change together; waves from these two sources illuminate the third screen. Bands parallel to the slits alternately dark and colored, are observed on this screen. These may be ex- plained immediately if light is due to wave- motion : because at points on the screen such that their distances to 0, and O, differ by half a wavelength or by an odd number of half wave- lengths there will be complete interference, and consequently darkness. Similarly at points on the screen whose distances from the slits O, and O, differ by a whole number of wave-lengths the two trains "of waves will reinforce each other, and there will be light. (It is evident that there is no destruction of energy — only a redistribution of it.) Referring to the figure, if P is a point in a bright band 0,P — 0,P = XX. where X is any integer number, 0, 1,2. 3, etc.. and X is the wave- lentrth of the waves. If the distance from the