Bob Evans, of Southland Boy's High School, asks :-
Bill Holvey, of Kirwee, asked:-
What causes the bright converging lines in the sky when looking away from a sunset?
John Campbell, a physicist at the University of Canterbury and who has an interest in optics in the atmosphere, responded.
This effect is due to the scattering of sunlight. They are called crepuscular rays, from the latin word for twilight, and sometimes in New Zealand the Ropes of Maui.
We see things because light reaches our eyes after being scattered off the object. In reading this page daylight or lamplight is scattered off the page into the eye where an image is formed on the light receptors at the back of the eyeball.
Tiny fluctuations in the density of air scatters sunlight. Blue light is scattered about 16 times more effectively than red light, hence our sky appears blue on days when the atmosphere is very clean, eg the day after rain. Dust particles, tiny water droplets, floating spider's webs etc in the atmosphere scatter white light so we see a general whiteness (a washed out blue at least) when looking at the sky on hazy days.
When the Sun is low, behind broken cloud or a mountain range, it only directly illuminates the Earth's atmosphere in bands passing through valleys or gaps in the cloud. We see bright rays where the sunlight is scattered from the regions of the atmosphere directly illuminated by the Sun. Between these bright rays the sky is darker as it is in shadow. But we do see some light from there because of light that has been scattered from the illuminated regions.
Due to the large distance between the Sun and the Earth, the rays are parallel lines but because of perspective they appear to us to be radiating from the Sun (if looking towards the sun) or towards the antisolar position (if looking away from the Sun.)
A related effect on a cloudless day when looking away from the Sun after sunset is the pinkish region of the sky above the bluish region, the boundary of which rises with time. Again the part of the sky we see as blue is due to light which has been scattered more than once on average. So the blue region is scattering from that part of the Earth's atmosphere which is in the Earth's shadow. The pink region is the part of the sky which is still illuminated directly by the Sun so there is incident red light left to be scattered to our eye.