[thedoc (OP)]

Angelo Reis  asked the Naked Scientists:
   Good day Dr Chris Smith,
My name is Angelo, I live in Johannesburg, South Africa; and am an avid listener of your fantastic segment on Talk Radio 702. I'm always fascinated about the questions and answers that are presented and try to listen to the show whenever I can.
I have a question of my own. Whenever I see some sort of blue light source - be it a blue LED or police lights on the road - I notice that the actual physical boundaries of the light source cannot be easily distinguishedalmost as if the light creates a sort of haze around the light source. I've also noticed that blue LED police lights inparticular have like a 'piercing' or focused effect on my vision similar to the effect a penlight laser pointer would have from a distance away. Why does this happen and why is it more prominent in blue/cyan light sources?
I'm looking forward to hearing from you soon.
Kinds Regards, Angelo

 
What do you think?

[Colin2B]

Angelo
Well observed.
The effect is called chromatic aberration, big name for a simple phenomenon. The lens of the eye isn't perfect and it focuses red and blue at slightly different points so when one is in focus the other isn't. Normally you don't notice it but if you look at a small red light eg standby light on the tv you will often see a blue halo, it's also noticeable with mercury vapour lights. The eye is focusing the red, but the blue is out of focus for I got the halo.
Hope that helps
Glad you enjoy the programmes.