Question: When some people have a catarax operation, can they see in ultra-violet?

This is a really interesting question!

This condition is known as aphakia. It occurs when people have a cataract operation and don’t have the lenses in their eyes replaced (here’s a good diagram with all the parts of the eye shown: http://www.nei.nih.gov/health/eyediagram/). The condition also be caused by damage to the eye, where the lense is either destroyed or has to be removed.

The lenses in our eyes protect them from UV damage by filtering it out. When those lenses are removed (say as part of a cataract operation) the UV light can be detected by the receptors in your eyes, and it apparently appears a violet colour. This is expected because UV light is at the ‘blue’ end of the light spectrum.

It is thought the famous painter Claude Monet had this condition after a cataract surgery, and that it affected the way he perceived colour, and this was why he painted things the way he did (http://en.wikipedia.org/wiki/Claude_Monet).