Optical Activity: How Do Organic Substances Rotate Polarized Light?

[alchemist]

optically active organic substances have the ability to rotate the plane of polarization of plane-polarized light. what i want to know is that how exactly do they rotate it?

 

[Tyger]

Here is how.

You'll probably find a better description in the Feynman Lectures, but I'll try to do it for you.

Plane polarized light can be represented as a superposition of two circularly polarized waves, a right handed and a left handed one. You can think of the waves as corkscrewing through space in a right or left handed fashion. When light passes through a material it is slowed down, the quantity that tells how much it is slowed down is called the "index of refraction". For opticaly active materials the index of refraction is different for right and left handed waves, so that when they are put together at the exit from the material to make a plane wave the plane of polarization will be rotated.

 

[ravenprp]

Optical activity in organic substances is a property that results from the structural asymmetry of molecules. This asymmetry is caused by the presence of chiral centers, which are carbon atoms bonded to four different groups. Due to this asymmetry, organic substances have the ability to rotate the plane of polarization of plane-polarized light.

When plane-polarized light passes through an optically active substance, the molecules in the substance interact with the electric field of the light. This interaction causes the molecules to rotate around the axis of light propagation, resulting in a change in the direction of the light's electric field. This change in direction is known as optical rotation.

The amount of rotation depends on the specific molecule and its concentration in the substance. The more chiral centers a molecule has, the greater its ability to rotate polarized light. Additionally, the concentration of the substance also plays a role in the degree of rotation.

It is important to note that only certain wavelengths of light are affected by optical activity. This is because the interaction between the molecules and the light depends on the wavelength of the light. This selective rotation of light is used in analytical techniques such as polarimetry to determine the concentration and identity of optically active substances.

In summary, the rotation of polarized light by organic substances is a result of the structural asymmetry of the molecules and their interaction with the electric field of light. This property is unique to optically active substances and is dependent on the molecule's structure and concentration.