More trivia:

Green sunshade and shadow

Observation:

There is a bus-stop with a green sunshade on Lancer’s Road that I pass every day. On sunny mornings, I noticed that the shadow under the sunshade was greenish. What was less clear was this: the green hue was most visible when I was some distance away and looking at the bus stop. But the closer I got to the bus stop, the fainter the green became – until it was barely noticeable when I was just under the sunshade. As I walked away from the bus stop, the same sequence occurred, in reverse. When I was again some distance away, the green shadow again became visible.

This puzzling green shadow did not occur on cloudy days, or in the afternoon. So clearly it had something to do with the direct sunshine falling on the sunshade, and not on diffuse scattered sunlight.

The 'sunshade' is standard green fibreglass.

Possible explanation:

a) the fibreglass looks green in reflection because all other colours are absorbed.

b) in transmission, which is what gives the green shadow, the incident, unabsorbed green light is scattered.

When sunlight is incident vertically on the sunshade, it traverses the minimum thickness t of the sunshade.

When the ray is incident at an angle q to the vertical, it passes through a thickness t /cos (q). This is, as expected, a minimum value t for q = 0, and increases as q increases.

So as the ray becomes more oblique, it passes through a greater thickness of the sunshade and becomes a darker green.

When I observe the ground under the sunshade from a distance, the rays that are visible to the eye are oblique and dark green, and when I am directly under the sunshade, the rays are vertical, and a very pale green.

This assumes that the sunshade is made of a homogenous translucent material and that it is optically isotropic.

According to the link:

http://aces.nmsu.edu/pubs/_circulars/circ556.html

polyethylene and fiberglass tend to scatter light, while acrylic and polycarbonate tend to allow radiation to pass through directly.

The angle of the ray determines the amount of scattering (because of t/cos(q)) and the more intense green occurs at oblique angles of incidence.

The amount of light transmitted through the sunshade is given by:

t = exp(- (α_s+ α_a) l )

where l = t/cos(q) is the path-length of the ray, α_s is the scattering coefficient and α_a is the absorption coefficient. For green, α_a = 0, and α_s ¹0, while for other colours in the visible α_a ¹0.

The scattering through the sunshade is exactly analogous to the enhancement of the redness of sunset rays as the sun dips below the horizon, due to the increased path-length of the oblique sun-rays.