What Is Polarimetry?
Polarimetry is the technique of measuring the 'polarization' of light. Most of the light we encounter every day is a chaotic mixture of light waves vibrating in all directions. Such a combination is known as 'unpolarized' light. When you turn on a lamp, for example, the light waves vibrate in all directions: up and down, side to side, or at any angle perpendicular (at right angles) to the direction the light wave is traveling out from the bulb.
If the light passes through certain materials or is reflected, the waves will tend to vibrate in only one direction and the light is said to be 'polarized'. Some materials contain long molecules which are lined up, like the slats on a wooden fence. As the light passes through this material, some of waves can pass through the slats, while others cannot. It's like trying to put a letter in a mail slot - the letter has to be lined up just right in order to get through the slot. By determining the amount and direction of polarization and how these change with wavelength, scientists can learn about what causes the energy to become polarized.
You can observe polarized light yourself by looking through a pair of polarizing sunglasses at the brightness of the blue sky about 90 degrees from the Sun (if the Sun is in the East or West, look North or South). As you rotate the glasses, the brightness of the sky will vary because the light has been polarized by being reflected in the atmosphere.
NASA Marshall Space Flight Center