Why Red??

[island]

The white light from a lighthouse sometimes was red when observed from a distance. There were instances of standings and sometimes wrecks that resulted from this. Why did the white light change to red?

[Kevin vk2ce]

White light contains all the colours of the rainbow and red has the longest wavelength so I guess at a distance red is the only colour left. Or something like that.

[island]

Quite likely this might have been appropriate to have been posted in the trivia forum.

You are correct. When there is a presence of very small particles of matter in the atmosphere, these tiny things cause the short wavelength blue to "scatter" followed by scattering of the longer wavelength yellow thus at a distance only the longest wavelength red with perhaps a bit of yellow remains. This same phenomena may be observed as red sky at dawn and at dusk. Further to this the scattered blue from the high altitudes is observed from the ground as blue sky. The degree of redness depends on the amount of particles present and the distance viewed from the source of the light.

With larger particles of moisture present in the atmosphere the colors are uniformly scattered thus causing mist and fog and clouds to appear white. The scattering of light from the upper atmosphere produces ambient light on the shade side of structures and the twilight before sunrise and after sunset. It is the small button-size ambient light sensor that has replaced lightkeepers.

With no atmosphere as on the moon the shadows are totally black. A sensor would not work so a lighthouse there would remain lit 24/7. But this would be a minor concern for the Coast Guard Aids-to-Nav team responsible for conducting routine inspection and maintenance.

[Kevin vk2ce]

Here endeth today's trivia lesson. Interesting stuff Dave. I did some further snooping on the topic and found some interesting info. Nothing to do with lighthouses though.