S. J. Gill University
of Colorado Boulder
I
I
A Demonstration of Optical Rotation with an "Eskimo
In 1822 A. Fresnell explained optical rotation by representing plane polarized light as a combination of two circularly polarized waves. A rotation of the plane, defined by the components of the left and right circularly polarized waves, occurs when one of the waves is slowed with respect to the other. If one wave is retarded a full wavelength in traveling through an optically active medium of length 1, then the plane of linear polarization is rotated rr radians. In general, t.he difference of the number of waves of left and right circular light of wavelengths X I and X, will be l I ( l / X , ) - ( l / X , ) ] ,so the left angular rotation of the plane is 0
= l ~ [ ( l / A d - (l/h,)]
This rotation is often expressed in terms of the refractive indices n, and n, for the respective waves. Since the wavelength is inversely proportional to the refractive index and directly proportional to the wavelength in vacuo, the rotation is given by
A demonstration of these concepts can be performed with a toy called the "Eskimo yo-yo."Z I t consists of
Figure 1. 1llu.trotion of "Eskimo yo-yo." lo providesynchronous rotation of bolls.
YO-YO"
string. Both balls are swung in opposite directions, and by moving the handle in an up and down motion, the balls can be made to travel a circular path in perfect synchrony (see Figure 1). The synchronous rotation of the balls illustrates the effect of the two circularly polarized beams of light. The up and down motion of the hand defines the direction of linear polarization. A feeling for this plane is particularly evident from the action of the toy. To represent the forward motion of a beam of plane polarized light, the operator of the yo-yo walks forward so the rotating balls trace two helical paths. The effect of optical rotation may be demonstrated by slowing the rotation of one ball for a moment so that the linear action of the hand is rotated to another angle. This works best going from an up and down motion to a horizontal motion, which corresponds to an opt,ical rotation of 90'. A45' rotation is shown in Figure 2. Occasionally the balls collide, particularly during the demonstration of optical rotation. This catast,rophe seems to add to the class's enjoyment of the demonstration, mostly by way of the lecturer's plight. I t is only fair to say that some practice is needed t o operate an "Eskimo yo-yo," hut the results seem to be worth the trouble.
Handle is moved vp and down
two rubber balls, each attached to one end of a five-foot plastic string. A handle is fixed near the middle of the 'FRESXEL, A,, "Oeuvres completes d'Augustin Fresnel," 1, iyo,~ ~ ~ 1 lSz2, 1 1 pp, , 731-51; see also L ~ ~ , T, . , M,, t o ~ ~ ~ i Figure ~ ~ 2.l Plone polorired light formed from two in-phase circular components. Circul=r components are illustrated b y rototing balls. Top Rgures Rotatory Power,,, Green and Co,, Ine,, New York, hdicote the state of vertical poiorirotion. Bonomfigurer indicate polarize1935, pp. 13-20. lion at -45' ond may be obtained from the vertical state when the cow= A model called the "Eskimo Go" is manufactured bv Creative lerclockwiie component speed-up of 9 0 0 ~ i t h Capers, Clsremant, California. I s the c!ackwire component.
Volume
38, Number 5, May 1961
/
263
