Physics Stuff
Odds and Ends for the Classroom and Beyond
Some answers, useful resources and more questions.
The left-handed Signature in our Flesh and Optical Rotation
Given the linear nature of Maxwell’s field equations, linearly polarized radiation can be described as composed of both right-circularly polarized and left-circularly polarized radiation. When such radiation traverses a medium that is chiral in the sense that it is made of chiral molecules, one of the circularly polarized components is propagated less rapidly than the other and the plane of the polarized light beam is rotated. This phenomenon is called optical rotation.
Chirality of Molecules
We describe a “chiral" molecule as one that possesses a handedness in the sense that it is not superimposable with its mirror image. So, just as left and right hands have thumb and fingers in the same order are mirror images but (arguably functionally) not the same, chiral molecules are molecule pairs that have the same atoms attached in a complimentary configuration, but which are mirror images and thus, not deemed identical.
The Helix Fields of Circularly Polarised Light
An ordinary incandescent bulb (just like the Sun’s radiation) emits (many wave packets of) photons pointing in random directions so its radiation is not said to be Polarised in any one direction.By definition its direction of polarization is defined to be the direction in which the electric field is oscillating up and down. In a wave description of circularly polarised light while looking at the source, the vector of (just) the electric field part of the light coming toward you appears to be rotating counterclockwise. In such right-circularly polarized the electric field vector (and it's complimentary magnetic field vector) of circularly polarized radiation describes a helix.
What is handedness of Spiral Staircases
Consider the choice of handedness of the spiral staircases in medieval castles that served best as a means to defend. They were (almost) always built with the spiral going in the same direction according to the right hand screw rule as one looks down the stairs from above so that the defending right- handed swordsman, who would either be coming down the stairs or backing up in reverse, could freely swing his sword. The attacking swordsman (ascending the stairs), unless left-handed would have his swing blocked by the wall. As such only a army of left-handed soldiers will have parity in the battle. Of course if they happen to succeed they will be at a disadvantage in defending the castle against another horde of lefties and be afforded no advantage over subsequent right-handed foes.
Parity Transformation
Unlike the inversion of time there is a real effect of inverting the sign of spatial directions on the description of the light ray. Such an inversion operation is called a Parity transformation.
The inversion of space transforms right-circularly polarized radiation into left-circularly polarized radiation. As such the Helicity of radiation flips sign with a parity transformation and thus we can assert that circularly polarized radiation possesses a handedness called chirality.