Dirty windscreen

[ll_billym]

I ran out of screenwash this morning and had dirty water from the road on my windscreen when I parked at work. When I drove home it was dry and the water had dried leaving dust on the windscreen, I still had no screenwash and put the wipers on, this cleared it a bit but all the street and car lights left "tracers" on the window, all bending and ending up in the bottom right of my windscreen. Sorry about the essay but I had to set the scene, anyway.....

1) Why does this happen?

2) What is the scientific term for this?

3) Why do all the tracers finish up in the same place?

4) Why do the tracers go "up" as well as finishing in the bottom right of the windwcreen.

5) Where can I get some cheap screenwash?

Thanks.

[dynamicduo]

Don't really know so this might be a silly answer. Waited a longtime and no answers posted so I felt sorry for you!

With a clean windscreen most light passes through the glass and some enters the eye. With a dirty screen, some of the dirt will cause refraction of the light (as would imperfections in the glass) and this causes extra light to enter the eye from certain angles. The brain will interpret this extra light as a beam of light coming from a straight line direction likely to be towards the bottom of the screen (brain can't appreciate that light can bend -- mirage effect)

Halfords do a good 'ready to use' screenwash. Not the cheapest but very effective and smells nice

[ll_billym]

Felt sorry for me, aahh.

Thanks for the answer, it sounds plausible enough.

I'm just off to Halfords...

Cya...