I came across this recently while researching a lens purchase. Tho, I seem to recall this from some other reading some time ago.
Much like leaded crystal glass, adding radioactive elements like thorium will increase the refractive index of the glass.
Anyway, some lenses of this vintage can have a yellow tint to the light going thru the the lens -- (not the reflection off the glass) This yellowness can be removed by laying the lens in the sun for a few days -- or heating it as one link claims.
Anyway, It looks like my Yashinon 55mm f/1.8 is radioactive, as will be my new Yashinon-DX f/1.4 that will be coming the mail soon -- can't wait to try that one out.
I haven't done much research into Pentax lenses, but some of those too are effected by this particularity of photography.
Here are some links for your curiosity if you're into this kinda thing.
Translated version of http://photo.6six6.de/Yashica_Objektiv_Radioaktivitaet.html 090.5 radioactive lenses pentax - Google Search