1) Is it permissible to wear transition eyeglass lenses on shabbat or Yom Tov? these lenses turn into sunglasses automatically when exposed to the UV rays from the sun and lighten up to clear lenses when inside. I think it is a chemical process. 2) Also does it make a difference if they are prescription lenses? 3) Is one permitted to wear nonprescription sunglasses where there is no Eruv? How about bifocals if the top is clear and only the bottom has a reading prescription?
Transition lenses – can be used on Shabbat [Shmirat Shabbat Kehilchata p.209] since there is no intention to change colour, there is no act on behalf of the person and the change is temporary. Prescription lenses – can be used on Shabbat. Non-prescription lenses – can't be worn out of the Eruv [ibid] Bifocal lenses – can be worn out of the Eruv. [ibid p. 210]