Why are there rings (halos) around street lights? Especially when it's foggy

Question

I was in a car that was turned off last night for some time and the windows became foggy via condensation (moisture droplets building up on one side of window). Looking outside, I could see that street lights which were near me had a halo or a ring around them. They would disappear if I wiped the moisture from the window. Why do I see halos around light through a foggy glass?

Also, after stepping outside I DID notice a ring around the street light, but it had a larger diameter and it was VERY faint. Why does this light effect occur even without a foggy window? What is going on?

Thank you.

Answer 1

The very fine droplets on the window act as a diffraction grating. In principle, a single droplet would produce (very faintly) something called an "Airy's disk" pattern. If they are all the same size, and the light is monochromatic, these will add constructively to make a clear ring. But in reality the droplets are many different sizes, and the light is not monochromatic. Consequently what you see is the sum of many of these patterns summed, each of a different size: this is the halo you see.

The faint ring you saw when you stepped out of the car may have a different origin. It may be that you had very small droplets on your glasses (did you step out of an airconditioned car into a humid night?), or it may be there were other objects (small droplets from fog beginning to form?) that were generating this pattern. The smaller the spheres, the larger the ring. The more uniform the spheres, the more it looks like a ring rather than a disk.

Answer 2

A couple of years ago a little piece of debris met my left eye. Later that day at night I started to notice a halo around streetlights as seen by my left eye. After an eye doctor visit and some treatment and time, they were gone for good. That made me ask myself the same question.

The rings are not around the street lights but in our views of them. Nearby rays coming out from the lamp are focused by your eye lenses to form a point in your retina. The halo you see is formed by rays that otherwise weren't aimed at your retina, but stuff refracted them midways and redirected them to your retina.

This could be by the droplets of water in the moisture of your glass (that's why cleaning it removed the halo) or by any other thing in the path of the rays. It is axially symmetric because only the rays that make a specific angle with the central ray get refracted into your eyeball, the rays at other angles get refracted elsewhere. And the refraction index marks this angle, which together with distance will give the perceived radius of the rings or halo.

As an experiment next time you can try to occlude with a finger the lamp and see if the halos are still there. Do you expect them to still be there for a) you inside your car, foggy glass? b) you outside your car?