Raindrops and polarization | lightcolourvision.org

Polarization of electromagnetic waves refers to the geometrical orientation of their oscillations.

Polarization restricts the orientation of the oscillations of the electric field of electromagnetic waves to a single plane from the point of view of an observer. This phenomenon is known as plane polarization.

Plane polarization filters out all the waves where the electric field is not orientated with the plane from the point of view of an observer.
To visualize plane polarization, imagine trying to push a large sheet of card through a window fitted with close-fitting vertical bars.
Only by aligning the card with the slots between the bars can it pass through. Align the card at any other angle and its path is blocked.
Now substitute the alignment of the electric field of an electromagnetic wave for the sheet of card, and plane polarization for the bars on the window.
Polarizing lenses used in sunglasses rely on plane polarization. The polarizing plane is orientated horizontally and cuts out glare by blocking vertically aligned waves.
Plane polarization is one of the optical effects that account for the appearance of rainbows.
It is the position of each raindrop on the arc of a rainbow, with respect to the observer, that determines the angle of the polarizing plane.
Rainbows are typically 96% polarized.

Let’s take this one step at a time

Rainbows form in the presence of sunlight, raindrops and an observer, and involve a combination of refraction, reflection and chromatic dispersion.
It is during reflection off the back of a droplet that light becomes polarized with respect to an observer.
The rear hemisphere of a raindrop forms a concave mirror in which an observer sees a tiny reflection of the Sun.
As a rainbow forms, an image of the Sun forms in each and every raindrop and the ones in exactly the right place at the right time become visible to the observer.
The light reflected towards an observer is polarized on a plane bisecting each droplet and at a tangent to the arc of the rainbow.
The rear hemisphere of a raindrop is best thought of as the half of the raindrop opposite the observer and with the Sun at its centre.
Now recall that to see yourself in a normal flat mirrored surface it has to be aligned perpendicular to your eyes. Get it right and you see yourself right in the middle. If it’s not perpendicular, then you see your image off-centre because the mirror is not aligned with your eyes on either the horizontal or vertical planes.
The Sun appears right in the centre of every raindrop from the point of view of an observer only if it is in exactly the right position in the sky at the right time. In all other cases, the light is scattered in other directions.
Only rays that strike at the point where the horizontal and vertical planes intersect are reflected towards the observer. Rays that strike to the left or right or above/below the centre-point miss the observer.
The correct alignment of a raindrop involves the vertical axis of the rear hemisphere being at exactly 90⁰ with respect to the observer. In the case of a primary rainbow, the horizontal axis is titled downwards by approx. 20.5⁰.